Various text and string functions for formatting output in PHP. Including sanitize, encode, entity conversion, removing accents and more. Borrowed from the WordPress Formatting API.

<?php

// =============================================================================
// Source: https://developer.wordpress.org/reference/files/wp-includes/formatting.php/
// =============================================================================

/**
 * Set the mbstring internal encoding to a binary safe encoding when
 * func_overload is enabled.
 *
 * When mbstring.func_overload is in use for multi-byte encodings, the results
 * from strlen() and similar functions respect the utf8 characters, causing
 * binary data to return incorrect lengths.
 *
 * This function overrides the mbstring encoding to a binary-safe encoding, and
 * resets it to the users expected encoding afterwards through the
 * `reset_mbstring_encoding` function.
 *
 * It is safe to recursively call this function, however each
 * `mbstring_binary_safe_encoding()` call must be followed up with an equal number
 * of `reset_mbstring_encoding()` calls.
 *
 * @see reset_mbstring_encoding()
 *
 * @staticvar array $encodings
 * @staticvar bool  $overloaded
 *
 * @param bool $reset Optional. Whether to reset the encoding back to a
 *                    previously-set encoding. Default false.
 */
function mbstring_binary_safe_encoding($reset = false)
{
    static $encodings = array();
    static $overloaded = null;

    if (is_null($overloaded)) {
        $overloaded = function_exists('mb_internal_encoding') && (ini_get('mbstring.func_overload') & 2);
    }

    if (false === $overloaded) {
        return;
    }

    if (!$reset) {
        $encoding = mb_internal_encoding();
        array_push($encodings, $encoding);
        mb_internal_encoding('ISO-8859-1');
    }

    if ($reset && $encodings) {
        $encoding = array_pop($encodings);
        mb_internal_encoding($encoding);
    }
}

/**
 * Reset the mbstring internal encoding to a users previously set encoding.
 *
 * @see mbstring_binary_safe_encoding()
 */
function reset_mbstring_encoding()
{
    mbstring_binary_safe_encoding(true);
}

/**
 * Checks to see if a string is utf8 encoded.
 *
 * NOTE: This function checks for 5-Byte sequences, UTF8
 *       has Bytes Sequences with a maximum length of 4.
 *
 * @author bmorel at ssi dot fr (modified)
 *
 * @since 1.2.1
 *
 * @param string $str The string to be checked
 *
 * @return bool True if $str fits a UTF-8 model, false otherwise.
 */
function seems_utf8($str)
{
    mbstring_binary_safe_encoding();
    $length = strlen($str);
    reset_mbstring_encoding();

    for ($i = 0; $i < $length; ++$i) {
        $c = ord($str[$i]);
        if ($c < 0x80) {
            $n = 0;
        } // 0bbbbbbb
        elseif (($c & 0xE0) == 0xC0) {
            $n = 1;
        } // 110bbbbb
        elseif (($c & 0xF0) == 0xE0) {
            $n = 2;
        } // 1110bbbb
        elseif (($c & 0xF8) == 0xF0) {
            $n = 3;
        } // 11110bbb
        elseif (($c & 0xFC) == 0xF8) {
            $n = 4;
        } // 111110bb
        elseif (($c & 0xFE) == 0xFC) {
            $n = 5;
        } // 1111110b
        else {
            return false;
        } // Does not match any model
        for ($j = 0; $j < $n; ++$j) { // n bytes matching 10bbbbbb follow ?
            if ((++$i == $length) || ((ord($str[$i]) & 0xC0) != 0x80)) {
                return false;
            }
        }
    }

    return true;
}

/**
 * Converts all accent characters to ASCII characters.
 *
 * If there are no accent characters, then the string given is just returned.
 *
 * @param string $string Text that might have accent characters
 *
 * @return string Filtered string with replaced "nice" characters.
 */
function remove_accents($string)
{
    if (!preg_match('/[\x80-\xff]/', $string)) {
        return $string;
    }

    if (seems_utf8($string)) {
        $chars = array(
        // Decompositions for Latin-1 Supplement
        chr(194).chr(170) => 'a', chr(194).chr(186) => 'o',
        chr(195).chr(128) => 'A', chr(195).chr(129) => 'A',
        chr(195).chr(130) => 'A', chr(195).chr(131) => 'A',
        chr(195).chr(132) => 'A', chr(195).chr(133) => 'A',
        chr(195).chr(134) => 'AE', chr(195).chr(135) => 'C',
        chr(195).chr(136) => 'E', chr(195).chr(137) => 'E',
        chr(195).chr(138) => 'E', chr(195).chr(139) => 'E',
        chr(195).chr(140) => 'I', chr(195).chr(141) => 'I',
        chr(195).chr(142) => 'I', chr(195).chr(143) => 'I',
        chr(195).chr(144) => 'D', chr(195).chr(145) => 'N',
        chr(195).chr(146) => 'O', chr(195).chr(147) => 'O',
        chr(195).chr(148) => 'O', chr(195).chr(149) => 'O',
        chr(195).chr(150) => 'O', chr(195).chr(153) => 'U',
        chr(195).chr(154) => 'U', chr(195).chr(155) => 'U',
        chr(195).chr(156) => 'U', chr(195).chr(157) => 'Y',
        chr(195).chr(158) => 'TH', chr(195).chr(159) => 's',
        chr(195).chr(160) => 'a', chr(195).chr(161) => 'a',
        chr(195).chr(162) => 'a', chr(195).chr(163) => 'a',
        chr(195).chr(164) => 'a', chr(195).chr(165) => 'a',
        chr(195).chr(166) => 'ae', chr(195).chr(167) => 'c',
        chr(195).chr(168) => 'e', chr(195).chr(169) => 'e',
        chr(195).chr(170) => 'e', chr(195).chr(171) => 'e',
        chr(195).chr(172) => 'i', chr(195).chr(173) => 'i',
        chr(195).chr(174) => 'i', chr(195).chr(175) => 'i',
        chr(195).chr(176) => 'd', chr(195).chr(177) => 'n',
        chr(195).chr(178) => 'o', chr(195).chr(179) => 'o',
        chr(195).chr(180) => 'o', chr(195).chr(181) => 'o',
        chr(195).chr(182) => 'o', chr(195).chr(184) => 'o',
        chr(195).chr(185) => 'u', chr(195).chr(186) => 'u',
        chr(195).chr(187) => 'u', chr(195).chr(188) => 'u',
        chr(195).chr(189) => 'y', chr(195).chr(190) => 'th',
        chr(195).chr(191) => 'y', chr(195).chr(152) => 'O',
        // Decompositions for Latin Extended-A
        chr(196).chr(128) => 'A', chr(196).chr(129) => 'a',
        chr(196).chr(130) => 'A', chr(196).chr(131) => 'a',
        chr(196).chr(132) => 'A', chr(196).chr(133) => 'a',
        chr(196).chr(134) => 'C', chr(196).chr(135) => 'c',
        chr(196).chr(136) => 'C', chr(196).chr(137) => 'c',
        chr(196).chr(138) => 'C', chr(196).chr(139) => 'c',
        chr(196).chr(140) => 'C', chr(196).chr(141) => 'c',
        chr(196).chr(142) => 'D', chr(196).chr(143) => 'd',
        chr(196).chr(144) => 'D', chr(196).chr(145) => 'd',
        chr(196).chr(146) => 'E', chr(196).chr(147) => 'e',
        chr(196).chr(148) => 'E', chr(196).chr(149) => 'e',
        chr(196).chr(150) => 'E', chr(196).chr(151) => 'e',
        chr(196).chr(152) => 'E', chr(196).chr(153) => 'e',
        chr(196).chr(154) => 'E', chr(196).chr(155) => 'e',
        chr(196).chr(156) => 'G', chr(196).chr(157) => 'g',
        chr(196).chr(158) => 'G', chr(196).chr(159) => 'g',
        chr(196).chr(160) => 'G', chr(196).chr(161) => 'g',
        chr(196).chr(162) => 'G', chr(196).chr(163) => 'g',
        chr(196).chr(164) => 'H', chr(196).chr(165) => 'h',
        chr(196).chr(166) => 'H', chr(196).chr(167) => 'h',
        chr(196).chr(168) => 'I', chr(196).chr(169) => 'i',
        chr(196).chr(170) => 'I', chr(196).chr(171) => 'i',
        chr(196).chr(172) => 'I', chr(196).chr(173) => 'i',
        chr(196).chr(174) => 'I', chr(196).chr(175) => 'i',
        chr(196).chr(176) => 'I', chr(196).chr(177) => 'i',
        chr(196).chr(178) => 'IJ', chr(196).chr(179) => 'ij',
        chr(196).chr(180) => 'J', chr(196).chr(181) => 'j',
        chr(196).chr(182) => 'K', chr(196).chr(183) => 'k',
        chr(196).chr(184) => 'k', chr(196).chr(185) => 'L',
        chr(196).chr(186) => 'l', chr(196).chr(187) => 'L',
        chr(196).chr(188) => 'l', chr(196).chr(189) => 'L',
        chr(196).chr(190) => 'l', chr(196).chr(191) => 'L',
        chr(197).chr(128) => 'l', chr(197).chr(129) => 'L',
        chr(197).chr(130) => 'l', chr(197).chr(131) => 'N',
        chr(197).chr(132) => 'n', chr(197).chr(133) => 'N',
        chr(197).chr(134) => 'n', chr(197).chr(135) => 'N',
        chr(197).chr(136) => 'n', chr(197).chr(137) => 'N',
        chr(197).chr(138) => 'n', chr(197).chr(139) => 'N',
        chr(197).chr(140) => 'O', chr(197).chr(141) => 'o',
        chr(197).chr(142) => 'O', chr(197).chr(143) => 'o',
        chr(197).chr(144) => 'O', chr(197).chr(145) => 'o',
        chr(197).chr(146) => 'OE', chr(197).chr(147) => 'oe',
        chr(197).chr(148) => 'R', chr(197).chr(149) => 'r',
        chr(197).chr(150) => 'R', chr(197).chr(151) => 'r',
        chr(197).chr(152) => 'R', chr(197).chr(153) => 'r',
        chr(197).chr(154) => 'S', chr(197).chr(155) => 's',
        chr(197).chr(156) => 'S', chr(197).chr(157) => 's',
        chr(197).chr(158) => 'S', chr(197).chr(159) => 's',
        chr(197).chr(160) => 'S', chr(197).chr(161) => 's',
        chr(197).chr(162) => 'T', chr(197).chr(163) => 't',
        chr(197).chr(164) => 'T', chr(197).chr(165) => 't',
        chr(197).chr(166) => 'T', chr(197).chr(167) => 't',
        chr(197).chr(168) => 'U', chr(197).chr(169) => 'u',
        chr(197).chr(170) => 'U', chr(197).chr(171) => 'u',
        chr(197).chr(172) => 'U', chr(197).chr(173) => 'u',
        chr(197).chr(174) => 'U', chr(197).chr(175) => 'u',
        chr(197).chr(176) => 'U', chr(197).chr(177) => 'u',
        chr(197).chr(178) => 'U', chr(197).chr(179) => 'u',
        chr(197).chr(180) => 'W', chr(197).chr(181) => 'w',
        chr(197).chr(182) => 'Y', chr(197).chr(183) => 'y',
        chr(197).chr(184) => 'Y', chr(197).chr(185) => 'Z',
        chr(197).chr(186) => 'z', chr(197).chr(187) => 'Z',
        chr(197).chr(188) => 'z', chr(197).chr(189) => 'Z',
        chr(197).chr(190) => 'z', chr(197).chr(191) => 's',
        // Decompositions for Latin Extended-B
        chr(200).chr(152) => 'S', chr(200).chr(153) => 's',
        chr(200).chr(154) => 'T', chr(200).chr(155) => 't',
        // Euro Sign
        chr(226).chr(130).chr(172) => 'E',
        // GBP (Pound) Sign
        chr(194).chr(163) => '',
        // Vowels with diacritic (Vietnamese)
        // unmarked
        chr(198).chr(160) => 'O', chr(198).chr(161) => 'o',
        chr(198).chr(175) => 'U', chr(198).chr(176) => 'u',
        // grave accent
        chr(225).chr(186).chr(166) => 'A', chr(225).chr(186).chr(167) => 'a',
        chr(225).chr(186).chr(176) => 'A', chr(225).chr(186).chr(177) => 'a',
        chr(225).chr(187).chr(128) => 'E', chr(225).chr(187).chr(129) => 'e',
        chr(225).chr(187).chr(146) => 'O', chr(225).chr(187).chr(147) => 'o',
        chr(225).chr(187).chr(156) => 'O', chr(225).chr(187).chr(157) => 'o',
        chr(225).chr(187).chr(170) => 'U', chr(225).chr(187).chr(171) => 'u',
        chr(225).chr(187).chr(178) => 'Y', chr(225).chr(187).chr(179) => 'y',
        // hook
        chr(225).chr(186).chr(162) => 'A', chr(225).chr(186).chr(163) => 'a',
        chr(225).chr(186).chr(168) => 'A', chr(225).chr(186).chr(169) => 'a',
        chr(225).chr(186).chr(178) => 'A', chr(225).chr(186).chr(179) => 'a',
        chr(225).chr(186).chr(186) => 'E', chr(225).chr(186).chr(187) => 'e',
        chr(225).chr(187).chr(130) => 'E', chr(225).chr(187).chr(131) => 'e',
        chr(225).chr(187).chr(136) => 'I', chr(225).chr(187).chr(137) => 'i',
        chr(225).chr(187).chr(142) => 'O', chr(225).chr(187).chr(143) => 'o',
        chr(225).chr(187).chr(148) => 'O', chr(225).chr(187).chr(149) => 'o',
        chr(225).chr(187).chr(158) => 'O', chr(225).chr(187).chr(159) => 'o',
        chr(225).chr(187).chr(166) => 'U', chr(225).chr(187).chr(167) => 'u',
        chr(225).chr(187).chr(172) => 'U', chr(225).chr(187).chr(173) => 'u',
        chr(225).chr(187).chr(182) => 'Y', chr(225).chr(187).chr(183) => 'y',
        // tilde
        chr(225).chr(186).chr(170) => 'A', chr(225).chr(186).chr(171) => 'a',
        chr(225).chr(186).chr(180) => 'A', chr(225).chr(186).chr(181) => 'a',
        chr(225).chr(186).chr(188) => 'E', chr(225).chr(186).chr(189) => 'e',
        chr(225).chr(187).chr(132) => 'E', chr(225).chr(187).chr(133) => 'e',
        chr(225).chr(187).chr(150) => 'O', chr(225).chr(187).chr(151) => 'o',
        chr(225).chr(187).chr(160) => 'O', chr(225).chr(187).chr(161) => 'o',
        chr(225).chr(187).chr(174) => 'U', chr(225).chr(187).chr(175) => 'u',
        chr(225).chr(187).chr(184) => 'Y', chr(225).chr(187).chr(185) => 'y',
        // acute accent
        chr(225).chr(186).chr(164) => 'A', chr(225).chr(186).chr(165) => 'a',
        chr(225).chr(186).chr(174) => 'A', chr(225).chr(186).chr(175) => 'a',
        chr(225).chr(186).chr(190) => 'E', chr(225).chr(186).chr(191) => 'e',
        chr(225).chr(187).chr(144) => 'O', chr(225).chr(187).chr(145) => 'o',
        chr(225).chr(187).chr(154) => 'O', chr(225).chr(187).chr(155) => 'o',
        chr(225).chr(187).chr(168) => 'U', chr(225).chr(187).chr(169) => 'u',
        // dot below
        chr(225).chr(186).chr(160) => 'A', chr(225).chr(186).chr(161) => 'a',
        chr(225).chr(186).chr(172) => 'A', chr(225).chr(186).chr(173) => 'a',
        chr(225).chr(186).chr(182) => 'A', chr(225).chr(186).chr(183) => 'a',
        chr(225).chr(186).chr(184) => 'E', chr(225).chr(186).chr(185) => 'e',
        chr(225).chr(187).chr(134) => 'E', chr(225).chr(187).chr(135) => 'e',
        chr(225).chr(187).chr(138) => 'I', chr(225).chr(187).chr(139) => 'i',
        chr(225).chr(187).chr(140) => 'O', chr(225).chr(187).chr(141) => 'o',
        chr(225).chr(187).chr(152) => 'O', chr(225).chr(187).chr(153) => 'o',
        chr(225).chr(187).chr(162) => 'O', chr(225).chr(187).chr(163) => 'o',
        chr(225).chr(187).chr(164) => 'U', chr(225).chr(187).chr(165) => 'u',
        chr(225).chr(187).chr(176) => 'U', chr(225).chr(187).chr(177) => 'u',
        chr(225).chr(187).chr(180) => 'Y', chr(225).chr(187).chr(181) => 'y',
        // Vowels with diacritic (Chinese, Hanyu Pinyin)
        chr(201).chr(145) => 'a',
        // macron
        chr(199).chr(149) => 'U', chr(199).chr(150) => 'u',
        // acute accent
        chr(199).chr(151) => 'U', chr(199).chr(152) => 'u',
        // caron
        chr(199).chr(141) => 'A', chr(199).chr(142) => 'a',
        chr(199).chr(143) => 'I', chr(199).chr(144) => 'i',
        chr(199).chr(145) => 'O', chr(199).chr(146) => 'o',
        chr(199).chr(147) => 'U', chr(199).chr(148) => 'u',
        chr(199).chr(153) => 'U', chr(199).chr(154) => 'u',
        // grave accent
        chr(199).chr(155) => 'U', chr(199).chr(156) => 'u',
        );

        // Used for locale-specific rules
        // $locale = get_locale();
        // $locale = 'en_US';

        // if ('de_DE' == $locale || 'de_DE_formal' == $locale) {
        //     $chars[ chr(195).chr(132) ] = 'Ae';
        //     $chars[ chr(195).chr(164) ] = 'ae';
        //     $chars[ chr(195).chr(150) ] = 'Oe';
        //     $chars[ chr(195).chr(182) ] = 'oe';
        //     $chars[ chr(195).chr(156) ] = 'Ue';
        //     $chars[ chr(195).chr(188) ] = 'ue';
        //     $chars[ chr(195).chr(159) ] = 'ss';
        // } elseif ('da_DK' === $locale) {
        //     $chars[ chr(195).chr(134) ] = 'Ae';
        //     $chars[ chr(195).chr(166) ] = 'ae';
        //     $chars[ chr(195).chr(152) ] = 'Oe';
        //     $chars[ chr(195).chr(184) ] = 'oe';
        //     $chars[ chr(195).chr(133) ] = 'Aa';
        //     $chars[ chr(195).chr(165) ] = 'aa';
        // }

        $string = strtr($string, $chars);
    } else {
        $chars = array();
        // Assume ISO-8859-1 if not UTF-8
        $chars['in'] = chr(128).chr(131).chr(138).chr(142).chr(154).chr(158)
            .chr(159).chr(162).chr(165).chr(181).chr(192).chr(193).chr(194)
            .chr(195).chr(196).chr(197).chr(199).chr(200).chr(201).chr(202)
            .chr(203).chr(204).chr(205).chr(206).chr(207).chr(209).chr(210)
            .chr(211).chr(212).chr(213).chr(214).chr(216).chr(217).chr(218)
            .chr(219).chr(220).chr(221).chr(224).chr(225).chr(226).chr(227)
            .chr(228).chr(229).chr(231).chr(232).chr(233).chr(234).chr(235)
            .chr(236).chr(237).chr(238).chr(239).chr(241).chr(242).chr(243)
            .chr(244).chr(245).chr(246).chr(248).chr(249).chr(250).chr(251)
            .chr(252).chr(253).chr(255);

        $chars['out'] = 'EfSZszYcYuAAAAAACEEEEIIIINOOOOOOUUUUYaaaaaaceeeeiiiinoooooouuuuyy';

        $string = strtr($string, $chars['in'], $chars['out']);
        $double_chars = array();
        $double_chars['in'] = array(chr(140), chr(156), chr(198), chr(208), chr(222), chr(223), chr(230), chr(240), chr(254));
        $double_chars['out'] = array('OE', 'oe', 'AE', 'DH', 'TH', 'ss', 'ae', 'dh', 'th');
        $string = str_replace($double_chars['in'], $double_chars['out'], $string);
    }

    return $string;
}

/**
 * Normalize EOL characters and strip duplicate whitespace.
 *
 * @param string $str The string to normalize.
 *
 * @return string The normalized string.
 */
function normalize_whitespace($str)
{
    $str = trim($str);
    $str = str_replace("\r", "\n", $str);
    $str = preg_replace(array('/\n+/', '/[ \t]+/'), array("\n", ' '), $str);

    return $str;
}

/**
 * Properly strip all HTML tags including script and style.
 *
 * This differs from strip_tags() because it removes the contents of
 * the `<script>` and `<style>` tags. E.g. `strip_tags('<script>something</script>')`
 * will return 'something'. strip_all_tags will return ''
 *
 * @param string $string       String containing HTML tags.
 * @param bool   $removeBreaks Optional. Whether to remove left over line breaks
 *                             and white space chars.
 *
 * @return string The processed string.
 */
function strip_all_tags($string, $removeBreaks = false)
{
    $string = preg_replace('@<(script|style)[^>]*?>.*?</\\1>@si', '', $string);
    $string = strip_tags($string);

    if ($removeBreaks) {
        $string = preg_replace('/[\r\n\t ]+/', ' ', $string);
    }

    return trim($string);
}

/**
 * Add leading zeros when necessary.
 *
 * If you set the threshold to '4' and the number is '10', then you will get
 * back '0010'. If you set the threshold to '4' and the number is '5000', then
 * you will get back '5000'.
 *
 * Uses sprintf to append the amount of zeros based on the $threshold parameter
 * and the size of the number. If the number is large enough, then no zeros will
 * be appended.
 *
 * @param int $number    Number to append zeros to if not greater than threshold.
 * @param int $threshold Digit places number needs to be to not have zeros added.
 *
 * @return string Adds leading zeros to number if needed.
 */
function zeroise($number, $threshold)
{
    return sprintf('%0'.$threshold.'s', $number);
}

/**
 * Converts invalid Unicode references range to valid range.
 *
 * @param string $content String with entities that need converting.
 *
 * @return string Converted string.
 */
function convert_invalid_entities($content)
{
    $htmltranswinuni = array(
        '&#128;' => '&#8364;', // the Euro sign
        '&#129;' => '',
        '&#130;' => '&#8218;', // these are Windows CP1252 specific characters
        '&#131;' => '&#402;',  // they would look weird on non-Windows browsers
        '&#132;' => '&#8222;',
        '&#133;' => '&#8230;',
        '&#134;' => '&#8224;',
        '&#135;' => '&#8225;',
        '&#136;' => '&#710;',
        '&#137;' => '&#8240;',
        '&#138;' => '&#352;',
        '&#139;' => '&#8249;',
        '&#140;' => '&#338;',
        '&#141;' => '',
        '&#142;' => '&#381;',
        '&#143;' => '',
        '&#144;' => '',
        '&#145;' => '&#8216;',
        '&#146;' => '&#8217;',
        '&#147;' => '&#8220;',
        '&#148;' => '&#8221;',
        '&#149;' => '&#8226;',
        '&#150;' => '&#8211;',
        '&#151;' => '&#8212;',
        '&#152;' => '&#732;',
        '&#153;' => '&#8482;',
        '&#154;' => '&#353;',
        '&#155;' => '&#8250;',
        '&#156;' => '&#339;',
        '&#157;' => '',
        '&#158;' => '&#382;',
        '&#159;' => '&#376;',
    );

    if (strpos($content, '&#1') !== false) {
        $content = strtr($content, $htmltranswinuni);
    }

    return $content;
}

/**
 * Sanitizes an HTML classname to ensure it only contains valid characters.
 *
 * Strips the string down to A-Z,a-z,0-9,_,-. If this results in an empty
 * string then it will return the alternative value supplied.
 *
 * @todo Expand to support the full range of CDATA that a class attribute
 *       can contain.
 *
 * @param string $class    The classname to be sanitized.
 * @param string $fallback Optional. The value to return if the sanitization
 *                         ends up as an empty string. Defaults to an empty
 *                         string.
 *
 * @return string The sanitized value
 */
function sanitize_html_class($class, $fallback = '')
{
    //Strip out any % encoded octets
    $sanitized = preg_replace('|%[a-fA-F0-9][a-fA-F0-9]|', '', $class);

    //Limit to A-Z,a-z,0-9,_,-
    $sanitized = preg_replace('/[^A-Za-z0-9_-]/', '', $sanitized);

    if ('' == $sanitized && $fallback) {
        return sanitize_html_class($fallback);
    }
}

/**
 * Sanitizes a string key.
 *
 * Keys are used as internal identifiers. Lowercase alphanumeric characters,
 * dashes and underscores are allowed.
 *
 * @param string $key The String key.
 *
 * @return string The Sanitized key.
 */
function sanitize_key($key)
{
    $raw_key = $key;
    $key = strtolower($key);
    $key = preg_replace('/[^a-z0-9_\-]/', '', $key);
}

/**
 * Sanitizes a username, stripping out unsafe characters.
 *
 * Removes tags, octets, entities, and if strict is enabled, will only keep
 * alphanumeric, _, space, ., -, @. After sanitizing, it passes the username,
 * raw username (the username in the parameter), and the value of $strict as
 * parameters for the 'sanitize_user' filter.
 *
 * @param string $username The username to be sanitized.
 * @param bool $strict If set limits $username to specific characters.
 *                     Default false.
 * @return string The sanitized username, after passing through filters.
 */
function sanitize_user($username, $strict = false)
{
    $raw_username = $username;
    $username = strip_all_tags($username);
    $username = remove_accents($username);
    // Kill octets
    $username = preg_replace('|%([a-fA-F0-9][a-fA-F0-9])|', '', $username);
    $username = preg_replace('/&.+?;/', '', $username); // Kill entities

    // If strict, reduce to ASCII for max portability.
    if ($strict) {
        $username = preg_replace('|[^a-z0-9 _.\-@]|i', '', $username);
    }

    $username = trim($username);
    // Consolidate contiguous whitespace
    $username = preg_replace('|\s+|', ' ', $username);
}

/**
 * Converts lone & characters into `&#038;` (a.k.a. `&amp;`).
 *
 * @param string $content String of characters to be converted.
 * @return string Converted string.
 */
function convert_chars($content)
{
    if (strpos($content, '&') !== false) {
        $content = preg_replace('/&([^#])(?![a-z1-4]{1,8};)/i', '&#038;$1', $content);
    }

    return $content;
}