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<?php

/**

* @file

* Provides Unicode-related conversions and operations.

*/

/**

 * Indicates an error during check for PHP unicode support.

 */

define('UNICODE_ERROR', -1);

/**

 * Indicates that standard PHP (emulated) unicode support is being used.

 */

define('UNICODE_SINGLEBYTE', 0);

/**

 * Indicates that full unicode support with the PHP mbstring extension is being

 * used.

 */

define('UNICODE_MULTIBYTE', 1);

/**

 * Matches Unicode characters that are word boundaries.

 *

 * Characters with the following General_category (gc) property values are used

 * as word boundaries. While this does not fully conform to the Word Boundaries

 * algorithm described in http://unicode.org/reports/tr29, as PCRE does not

 * contain the Word_Break property table, this simpler algorithm has to do.

 * - Cc, Cf, Cn, Co, Cs: Other.

 * - Pc, Pd, Pe, Pf, Pi, Po, Ps: Punctuation.

 * - Sc, Sk, Sm, So: Symbols.

 * - Zl, Zp, Zs: Separators.

 *

 * Non-boundary characters include the following General_category (gc) property

 * values:

 * - Ll, Lm, Lo, Lt, Lu: Letters.

 * - Mc, Me, Mn: Combining Marks.

 * - Nd, Nl, No: Numbers.

 *

 * Note that the PCRE property matcher is not used because we wanted to be

 * compatible with Unicode 5.2.0 regardless of the PCRE version used (and any

 * bugs in PCRE property tables).

 *

 * @see http://unicode.org/glossary

 */

define('PREG_CLASS_UNICODE_WORD_BOUNDARY',

  '\x{0}-\x{2F}\x{3A}-\x{40}\x{5B}-\x{60}\x{7B}-\x{A9}\x{AB}-\x{B1}\x{B4}' .

  '\x{B6}-\x{B8}\x{BB}\x{BF}\x{D7}\x{F7}\x{2C2}-\x{2C5}\x{2D2}-\x{2DF}' .

  '\x{2E5}-\x{2EB}\x{2ED}\x{2EF}-\x{2FF}\x{375}\x{37E}-\x{385}\x{387}\x{3F6}' .

  '\x{482}\x{55A}-\x{55F}\x{589}-\x{58A}\x{5BE}\x{5C0}\x{5C3}\x{5C6}' .

  '\x{5F3}-\x{60F}\x{61B}-\x{61F}\x{66A}-\x{66D}\x{6D4}\x{6DD}\x{6E9}' .

  '\x{6FD}-\x{6FE}\x{700}-\x{70F}\x{7F6}-\x{7F9}\x{830}-\x{83E}' .

  '\x{964}-\x{965}\x{970}\x{9F2}-\x{9F3}\x{9FA}-\x{9FB}\x{AF1}\x{B70}' .

  '\x{BF3}-\x{BFA}\x{C7F}\x{CF1}-\x{CF2}\x{D79}\x{DF4}\x{E3F}\x{E4F}' .

  '\x{E5A}-\x{E5B}\x{F01}-\x{F17}\x{F1A}-\x{F1F}\x{F34}\x{F36}\x{F38}' .

  '\x{F3A}-\x{F3D}\x{F85}\x{FBE}-\x{FC5}\x{FC7}-\x{FD8}\x{104A}-\x{104F}' .

  '\x{109E}-\x{109F}\x{10FB}\x{1360}-\x{1368}\x{1390}-\x{1399}\x{1400}' .

  '\x{166D}-\x{166E}\x{1680}\x{169B}-\x{169C}\x{16EB}-\x{16ED}' .

  '\x{1735}-\x{1736}\x{17B4}-\x{17B5}\x{17D4}-\x{17D6}\x{17D8}-\x{17DB}' .

  '\x{1800}-\x{180A}\x{180E}\x{1940}-\x{1945}\x{19DE}-\x{19FF}' .

  '\x{1A1E}-\x{1A1F}\x{1AA0}-\x{1AA6}\x{1AA8}-\x{1AAD}\x{1B5A}-\x{1B6A}' .

  '\x{1B74}-\x{1B7C}\x{1C3B}-\x{1C3F}\x{1C7E}-\x{1C7F}\x{1CD3}\x{1FBD}' .

  '\x{1FBF}-\x{1FC1}\x{1FCD}-\x{1FCF}\x{1FDD}-\x{1FDF}\x{1FED}-\x{1FEF}' .

  '\x{1FFD}-\x{206F}\x{207A}-\x{207E}\x{208A}-\x{208E}\x{20A0}-\x{20B8}' .

  '\x{2100}-\x{2101}\x{2103}-\x{2106}\x{2108}-\x{2109}\x{2114}' .

  '\x{2116}-\x{2118}\x{211E}-\x{2123}\x{2125}\x{2127}\x{2129}\x{212E}' .

  '\x{213A}-\x{213B}\x{2140}-\x{2144}\x{214A}-\x{214D}\x{214F}' .

  '\x{2190}-\x{244A}\x{249C}-\x{24E9}\x{2500}-\x{2775}\x{2794}-\x{2B59}' .

  '\x{2CE5}-\x{2CEA}\x{2CF9}-\x{2CFC}\x{2CFE}-\x{2CFF}\x{2E00}-\x{2E2E}' .

  '\x{2E30}-\x{3004}\x{3008}-\x{3020}\x{3030}\x{3036}-\x{3037}' .

  '\x{303D}-\x{303F}\x{309B}-\x{309C}\x{30A0}\x{30FB}\x{3190}-\x{3191}' .

  '\x{3196}-\x{319F}\x{31C0}-\x{31E3}\x{3200}-\x{321E}\x{322A}-\x{3250}' .

  '\x{3260}-\x{327F}\x{328A}-\x{32B0}\x{32C0}-\x{33FF}\x{4DC0}-\x{4DFF}' .

  '\x{A490}-\x{A4C6}\x{A4FE}-\x{A4FF}\x{A60D}-\x{A60F}\x{A673}\x{A67E}' .

  '\x{A6F2}-\x{A716}\x{A720}-\x{A721}\x{A789}-\x{A78A}\x{A828}-\x{A82B}' .

  '\x{A836}-\x{A839}\x{A874}-\x{A877}\x{A8CE}-\x{A8CF}\x{A8F8}-\x{A8FA}' .

  '\x{A92E}-\x{A92F}\x{A95F}\x{A9C1}-\x{A9CD}\x{A9DE}-\x{A9DF}' .

  '\x{AA5C}-\x{AA5F}\x{AA77}-\x{AA79}\x{AADE}-\x{AADF}\x{ABEB}' .

  '\x{E000}-\x{F8FF}\x{FB29}\x{FD3E}-\x{FD3F}\x{FDFC}-\x{FDFD}' .

  '\x{FE10}-\x{FE19}\x{FE30}-\x{FE6B}\x{FEFF}-\x{FF0F}\x{FF1A}-\x{FF20}' .

  '\x{FF3B}-\x{FF40}\x{FF5B}-\x{FF65}\x{FFE0}-\x{FFFD}');

/**

 * Wrapper around _unicode_check().

 */

function unicode_check() {

  list($GLOBALS['multibyte']) = _unicode_check();

}

/**

 * Perform checks about Unicode support in PHP, and set the right settings if

 * needed.

 *

 * Because Drupal needs to be able to handle text in various encodings, we do

 * not support mbstring function overloading. HTTP input/output conversion must

 * be disabled for similar reasons.

 *

 * @param $errors

 *   Whether to report any fatal errors with form_set_error().

 */

function _unicode_check() {

  // Ensure translations don't break during installation.

  $t = get_t();

  // Check for mbstring extension

  if (!function_exists('mb_strlen')) {

    return array(UNICODE_SINGLEBYTE, $t('Operations on Unicode strings are emulated on a best-effort basis. Install the <a href="@url">PHP mbstring extension</a> for improved Unicode support.', array('@url' => 'http://www.php.net/mbstring')));

  }

  // Check mbstring configuration

  if (ini_get('mbstring.func_overload') != 0) {

    return array(UNICODE_ERROR, $t('Multibyte string function overloading in PHP is active and must be disabled. Check the php.ini <em>mbstring.func_overload</em> setting. Please refer to the <a href="@url">PHP mbstring documentation</a> for more information.', array('@url' => 'http://www.php.net/mbstring')));

  }

  if (ini_get('mbstring.encoding_translation') != 0) {

    return array(UNICODE_ERROR, $t('Multibyte string input conversion in PHP is active and must be disabled. Check the php.ini <em>mbstring.encoding_translation</em> setting. Please refer to the <a href="@url">PHP mbstring documentation</a> for more information.', array('@url' => 'http://www.php.net/mbstring')));

  }

  // mbstring.http_input and mbstring.http_output are deprecated and empty by

  // default in PHP 5.6.

  if (version_compare(PHP_VERSION, '5.6.0') == -1) {

    if (ini_get('mbstring.http_input') != 'pass') {

      return array(UNICODE_ERROR, $t('Multibyte string input conversion in PHP is active and must be disabled. Check the php.ini <em>mbstring.http_input</em> setting. Please refer to the <a href="@url">PHP mbstring documentation</a> for more information.', array('@url' => 'http://www.php.net/mbstring')));

    }

    if (ini_get('mbstring.http_output') != 'pass') {

      return array(UNICODE_ERROR, $t('Multibyte string output conversion in PHP is active and must be disabled. Check the php.ini <em>mbstring.http_output</em> setting. Please refer to the <a href="@url">PHP mbstring documentation</a> for more information.', array('@url' => 'http://www.php.net/mbstring')));

    }

  }

  // Set appropriate configuration

  mb_internal_encoding('utf-8');

  mb_language('uni');

  return array(UNICODE_MULTIBYTE, '');

}

/**

 * Returns Unicode library status and errors.

 */

function unicode_requirements() {

  // Ensure translations don't break during installation.

  $t = get_t();

  $libraries = array(

    UNICODE_SINGLEBYTE => $t('Standard PHP'),

    UNICODE_MULTIBYTE => $t('PHP Mbstring Extension'),

    UNICODE_ERROR => $t('Error'),

  );

  $severities = array(

    UNICODE_SINGLEBYTE => REQUIREMENT_WARNING,

    UNICODE_MULTIBYTE => REQUIREMENT_OK,

    UNICODE_ERROR => REQUIREMENT_ERROR,

  );

  list($library, $description) = _unicode_check();

  $requirements['unicode'] = array(

    'title' => $t('Unicode library'),

    'value' => $libraries[$library],

  );

  if ($description) {

    $requirements['unicode']['description'] = $description;

  }

  $requirements['unicode']['severity'] = $severities[$library];

  return $requirements;

}

/**

 * Prepares a new XML parser.

 *

 * This is a wrapper around xml_parser_create() which extracts the encoding

 * from the XML data first and sets the output encoding to UTF-8. This function

 * should be used instead of xml_parser_create(), because PHP 4's XML parser

 * doesn't check the input encoding itself. "Starting from PHP 5, the input

 * encoding is automatically detected, so that the encoding parameter specifies

 * only the output encoding."

 *

 * This is also where unsupported encodings will be converted. Callers should

 * take this into account: $data might have been changed after the call.

 *

 * @param $data

 *   The XML data which will be parsed later.

 *

 * @return

 *   An XML parser object or FALSE on error.

 *

 * @ingroup php_wrappers

 */

function drupal_xml_parser_create(&$data) {

  // Default XML encoding is UTF-8

  $encoding = 'utf-8';

  $bom = FALSE;

  // Check for UTF-8 byte order mark (PHP5's XML parser doesn't handle it).

  if (!strncmp($data, "\xEF\xBB\xBF", 3)) {

    $bom = TRUE;

    $data = substr($data, 3);

  }

  // Check for an encoding declaration in the XML prolog if no BOM was found.

  if (!$bom && preg_match('/^<\?xml[^>]+encoding="(.+?)"/', $data, $match)) {

    $encoding = $match[1];

  }

  // Unsupported encodings are converted here into UTF-8.

  $php_supported = array('utf-8', 'iso-8859-1', 'us-ascii');

  if (!in_array(strtolower($encoding), $php_supported)) {

    $out = drupal_convert_to_utf8($data, $encoding);

    if ($out !== FALSE) {

      $encoding = 'utf-8';

      $data = preg_replace('/^(<\?xml[^>]+encoding)="(.+?)"/', '\\1="utf-8"', $out);

    }

    else {

      watchdog('php', 'Could not convert XML encoding %s to UTF-8.', array('%s' => $encoding), WATCHDOG_WARNING);

      return FALSE;

    }

  }

  $xml_parser = xml_parser_create($encoding);

  xml_parser_set_option($xml_parser, XML_OPTION_TARGET_ENCODING, 'utf-8');

  return $xml_parser;

}

/**

 * Converts data to UTF-8.

 *

 * Requires the iconv, GNU recode or mbstring PHP extension.

 *

 * @param $data

 *   The data to be converted.

 * @param $encoding

 *   The encoding that the data is in.

 *

 * @return

 *   Converted data or FALSE.

 */

function drupal_convert_to_utf8($data, $encoding) {

  if (function_exists('iconv')) {

    $out = @iconv($encoding, 'utf-8', $data);

  }

  elseif (function_exists('mb_convert_encoding')) {

    $out = @mb_convert_encoding($data, 'utf-8', $encoding);

  }

  elseif (function_exists('recode_string')) {

    $out = @recode_string($encoding . '..utf-8', $data);

  }

  else {

    watchdog('php', 'Unsupported encoding %s. Please install iconv, GNU recode or mbstring for PHP.', array('%s' => $encoding), WATCHDOG_ERROR);

    return FALSE;

  }

  return $out;

}

/**

 * Truncates a UTF-8-encoded string safely to a number of bytes.

 *

 * If the end position is in the middle of a UTF-8 sequence, it scans backwards

 * until the beginning of the byte sequence.

 *

 * Use this function whenever you want to chop off a string at an unsure

 * location. On the other hand, if you're sure that you're splitting on a

 * character boundary (e.g. after using strpos() or similar), you can safely

 * use substr() instead.

 *

 * @param $string

 *   The string to truncate.

 * @param $len

 *   An upper limit on the returned string length.

 *

 * @return

 *   The truncated string.

 */

function drupal_truncate_bytes($string, $len) {

  if (strlen($string) <= $len) {

    return $string;

  }

  if ((ord($string[$len]) < 0x80) || (ord($string[$len]) >= 0xC0)) {

    return substr($string, 0, $len);

  }

  // Scan backwards to beginning of the byte sequence.

  while (--$len >= 0 && ord($string[$len]) >= 0x80 && ord($string[$len]) < 0xC0);

  return substr($string, 0, $len);

}

/**

 * Truncates a UTF-8-encoded string safely to a number of characters.

 *

 * @param $string

 *   The string to truncate.

 * @param $max_length

 *   An upper limit on the returned string length, including trailing ellipsis

 *   if $add_ellipsis is TRUE.

 * @param $wordsafe

 *   If TRUE, attempt to truncate on a word boundary. Word boundaries are

 *   spaces, punctuation, and Unicode characters used as word boundaries in

 *   non-Latin languages; see PREG_CLASS_UNICODE_WORD_BOUNDARY for more

 *   information. If a word boundary cannot be found that would make the length

 *   of the returned string fall within length guidelines (see parameters

 *   $max_length and $min_wordsafe_length), word boundaries are ignored.

 * @param $add_ellipsis

 *   If TRUE, add t('...') to the end of the truncated string (defaults to

 *   FALSE). The string length will still fall within $max_length.

 * @param $min_wordsafe_length

 *   If $wordsafe is TRUE, the minimum acceptable length for truncation (before

 *   adding an ellipsis, if $add_ellipsis is TRUE). Has no effect if $wordsafe

 *   is FALSE. This can be used to prevent having a very short resulting string

 *   that will not be understandable. For instance, if you are truncating the

 *   string "See myverylongurlexample.com for more information" to a word-safe

 *   return length of 20, the only available word boundary within 20 characters

 *   is after the word "See", which wouldn't leave a very informative string. If

 *   you had set $min_wordsafe_length to 10, though, the function would realise

 *   that "See" alone is too short, and would then just truncate ignoring word

 *   boundaries, giving you "See myverylongurl..." (assuming you had set

 *   $add_ellipses to TRUE).

 *

 * @return string

 *   The truncated string.

 */

function truncate_utf8($string, $max_length, $wordsafe = FALSE, $add_ellipsis = FALSE, $min_wordsafe_length = 1) {

  $ellipsis = '';

  $max_length = max($max_length, 0);

  $min_wordsafe_length = max($min_wordsafe_length, 0);

  if (drupal_strlen($string) <= $max_length) {

    // No truncation needed, so don't add ellipsis, just return.

    return $string;

  }

  if ($add_ellipsis) {

    // Truncate ellipsis in case $max_length is small.

    $ellipsis = drupal_substr(t('...'), 0, $max_length);

    $max_length -= drupal_strlen($ellipsis);

    $max_length = max($max_length, 0);

  }

  if ($max_length <= $min_wordsafe_length) {

    // Do not attempt word-safe if lengths are bad.

    $wordsafe = FALSE;

  }

  if ($wordsafe) {

    $matches = array();

    // Find the last word boundary, if there is one within $min_wordsafe_length

    // to $max_length characters. preg_match() is always greedy, so it will

    // find the longest string possible.

    $found = preg_match('/^(.{' . $min_wordsafe_length . ',' . $max_length . '})[' . PREG_CLASS_UNICODE_WORD_BOUNDARY . ']/u', $string, $matches);

    if ($found) {

      $string = $matches[1];

    }

    else {

      $string = drupal_substr($string, 0, $max_length);

    }

  }

  else {

    $string = drupal_substr($string, 0, $max_length);

  }

  if ($add_ellipsis) {

    $string .= $ellipsis;

  }

  return $string;

}

/**

 * Encodes MIME/HTTP header values that contain incorrectly encoded characters.

 *

 * For example, mime_header_encode('tést.txt') returns "=?UTF-8?B?dMOpc3QudHh0?=".

 *

 * See http://www.rfc-editor.org/rfc/rfc2047.txt for more information.

 *

 * Notes:

 * - Only encode strings that contain non-ASCII characters.

 * - We progressively cut-off a chunk with truncate_utf8(). This is to ensure

 *   each chunk starts and ends on a character boundary.

 * - Using \n as the chunk separator may cause problems on some systems and may

 *   have to be changed to \r\n or \r.

 *

 * @param $string

 *   The header to encode.

 *

 * @return string

 *   The mime-encoded header.

 *

 * @see mime_header_decode()

 */

function mime_header_encode($string) {

  if (preg_match('/[^\x20-\x7E]/', $string)) {

    $chunk_size = 47; // floor((75 - strlen("=?UTF-8?B??=")) * 0.75);

    $len = strlen($string);

    $output = '';

    while ($len > 0) {

      $chunk = drupal_truncate_bytes($string, $chunk_size);

      $output .= ' =?UTF-8?B?' . base64_encode($chunk) . "?=\n";

      $c = strlen($chunk);

      $string = substr($string, $c);

      $len -= $c;

    }

    return trim($output);

  }

  return $string;

}

/**

 * Decodes MIME/HTTP encoded header values.

 *

 * @param $header

 *   The header to decode.

 *

 * @return string

 *   The mime-decoded header.

 *

 * @see mime_header_encode()

 */

function mime_header_decode($header) {

  // First step: encoded chunks followed by other encoded chunks (need to collapse whitespace)

  $header = preg_replace_callback('/=\?([^?]+)\?(Q|B)\?([^?]+|\?(?!=))\?=\s+(?==\?)/', '_mime_header_decode', $header);

  // Second step: remaining chunks (do not collapse whitespace)

  return preg_replace_callback('/=\?([^?]+)\?(Q|B)\?([^?]+|\?(?!=))\?=/', '_mime_header_decode', $header);

}

/**

 * Decodes encoded header data passed from mime_header_decode().

 *

 * Callback for preg_replace_callback() within mime_header_decode().

 *

 * @param $matches

 *   The array of matches from preg_replace_callback().

 *

 * @return string

 *   The mime-decoded string.

 *

 * @see mime_header_decode()

 */

function _mime_header_decode($matches) {

  // Regexp groups:

  // 1: Character set name

  // 2: Escaping method (Q or B)

  // 3: Encoded data

  $data = ($matches[2] == 'B') ? base64_decode($matches[3]) : str_replace('_', ' ', quoted_printable_decode($matches[3]));

  if (strtolower($matches[1]) != 'utf-8') {

    $data = drupal_convert_to_utf8($data, $matches[1]);

  }

  return $data;

}

/**

 * Decodes all HTML entities (including numerical ones) to regular UTF-8 bytes.

 *

 * Double-escaped entities will only be decoded once ("<" becomes "<"

 * , not "<"). Be careful when using this function, as decode_entities can

 * revert previous sanitization efforts (&lt;script&gt; will become <script>).

 *

 * @param $text

 *   The text to decode entities in.

 *

 * @return

 *   The input $text, with all HTML entities decoded once.

 */

function decode_entities($text) {

  return html_entity_decode($text, ENT_QUOTES, 'UTF-8');

}

/**

 * Counts the number of characters in a UTF-8 string.

 *

 * This is less than or equal to the byte count.

 *

 * @param $text

 *   The string to run the operation on.

 *

 * @return integer

 *   The length of the string.

 *

 * @ingroup php_wrappers

 */

function drupal_strlen($text) {

  global $multibyte;

  if ($multibyte == UNICODE_MULTIBYTE) {

    return mb_strlen($text);

  }

  else {

    // Do not count UTF-8 continuation bytes.

    return strlen(preg_replace("/[\x80-\xBF]/", '', $text));

  }

}

/**

 * Uppercase a UTF-8 string.

 *

 * @param $text

 *   The string to run the operation on.

 *

 * @return string

 *   The string in uppercase.

 *

 * @ingroup php_wrappers

 */

function drupal_strtoupper($text) {

  global $multibyte;

  if ($multibyte == UNICODE_MULTIBYTE) {

    return mb_strtoupper($text);

  }

  else {

    // Use C-locale for ASCII-only uppercase

    $text = strtoupper($text);

    // Case flip Latin-1 accented letters

    $text = preg_replace_callback('/\xC3[\xA0-\xB6\xB8-\xBE]/', '_unicode_caseflip', $text);

    return $text;

  }

}

/**

 * Lowercase a UTF-8 string.

 *

 * @param $text

 *   The string to run the operation on.

 *

 * @return string

 *   The string in lowercase.

 *

 * @ingroup php_wrappers

 */

function drupal_strtolower($text) {

  global $multibyte;

  if ($multibyte == UNICODE_MULTIBYTE) {

    return mb_strtolower($text);

  }

  else {

    // Use C-locale for ASCII-only lowercase

    $text = strtolower($text);

    // Case flip Latin-1 accented letters

    $text = preg_replace_callback('/\xC3[\x80-\x96\x98-\x9E]/', '_unicode_caseflip', $text);

    return $text;

  }

}

/**

 * Flips U+C0-U+DE to U+E0-U+FD and back.

 *

 * @param $matches

 *   An array of matches.

 *

 * @return array

 *   The Latin-1 version of the array of matches.

 *

 * @see drupal_strtolower()

 */

function _unicode_caseflip($matches) {

  return $matches[0][0] . chr(ord($matches[0][1]) ^ 32);

}

/**

 * Capitalizes the first letter of a UTF-8 string.

 *

 * @param $text

 *   The string to convert.

 *

 * @return

 *   The string with the first letter as uppercase.

 *

 * @ingroup php_wrappers

 */

function drupal_ucfirst($text) {

  // Note: no mbstring equivalent!

  return drupal_strtoupper(drupal_substr($text, 0, 1)) . drupal_substr($text, 1);

}

/**

 * Cuts off a piece of a string based on character indices and counts.

 *

 * Follows the same behavior as PHP's own substr() function. Note that for

 * cutting off a string at a known character/substring location, the usage of

 * PHP's normal strpos/substr is safe and much faster.

 *

 * @param $text

 *   The input string.

 * @param $start

 *   The position at which to start reading.

 * @param $length

 *   The number of characters to read.

 *

 * @return

 *   The shortened string.

 *

 * @ingroup php_wrappers

 */

function drupal_substr($text, $start, $length = NULL) {

  global $multibyte;

  if ($multibyte == UNICODE_MULTIBYTE) {

    return $length === NULL ? mb_substr($text, $start) : mb_substr($text, $start, $length);

  }

  else {

    $strlen = strlen($text);

    // Find the starting byte offset.

    $bytes = 0;

    if ($start > 0) {

      // Count all the continuation bytes from the start until we have found

      // $start characters or the end of the string.

      $bytes = -1; $chars = -1;

      while ($bytes < $strlen - 1 && $chars < $start) {

        $bytes++;

        $c = ord($text[$bytes]);

        if ($c < 0x80 || $c >= 0xC0) {

          $chars++;

        }

      }

    }

    elseif ($start < 0) {

      // Count all the continuation bytes from the end until we have found

      // abs($start) characters.

      $start = abs($start);

      $bytes = $strlen; $chars = 0;

      while ($bytes > 0 && $chars < $start) {

        $bytes--;

        $c = ord($text[$bytes]);

        if ($c < 0x80 || $c >= 0xC0) {

          $chars++;

        }

      }

    }

    $istart = $bytes;

    // Find the ending byte offset.

    if ($length === NULL) {

      $iend = $strlen;

    }

    elseif ($length > 0) {

      // Count all the continuation bytes from the starting index until we have

      // found $length characters or reached the end of the string, then

      // backtrace one byte.

      $iend = $istart - 1;

      $chars = -1;

      $last_real = FALSE;

      while ($iend < $strlen - 1 && $chars < $length) {

        $iend++;

        $c = ord($text[$iend]);

        $last_real = FALSE;

        if ($c < 0x80 || $c >= 0xC0) {

          $chars++;

          $last_real = TRUE;

        }

      }

      // Backtrace one byte if the last character we found was a real character

      // and we don't need it.

      if ($last_real && $chars >= $length) {

        $iend--;

      }

    }

    elseif ($length < 0) {

      // Count all the continuation bytes from the end until we have found

      // abs($start) characters, then backtrace one byte.

      $length = abs($length);

      $iend = $strlen; $chars = 0;

      while ($iend > 0 && $chars < $length) {

        $iend--;

        $c = ord($text[$iend]);

        if ($c < 0x80 || $c >= 0xC0) {

          $chars++;

        }

      }

      // Backtrace one byte if we are not at the beginning of the string.

      if ($iend > 0) {

        $iend--;

      }

    }

    else {

      // $length == 0, return an empty string.

      return '';

    }

    return substr($text, $istart, max(0, $iend - $istart + 1));

  }

}