Convert national chars into their latin equivalents in PHP

For Brazilian Portuguese I use the following

$string = 'tranformação';
$search = array('/é/', '/ç/', '/ã/', '/á/', '/ó/', '/ã/', '/ó/', '/ú/');
$replace = array('e', 'c', 'a', 'a', 'o', 'a', 'o', 'u');
$new_string = preg_replace($search, $replace, $string);
echo $new_string;

You will need to provide the characters in both upper and lower case to meet your needs.

It obviously does not cover every single character, but should help with some of the more common ones:

<?php
/**
 * Replaces special characters in a string with their "non-special" counterpart.
 *
 * Useful for friendly URLs.
 *
 * @access public
 * @param string
 * @return string
 */
function convertAccentsAndSpecialToNormal($string) {
    $table = array(
        'À'=>'A', 'Á'=>'A', 'Â'=>'A', 'Ã'=>'A', 'Ä'=>'A', 'Å'=>'A', 'Ă'=>'A', 'Ā'=>'A', 'Ą'=>'A', 'Æ'=>'A', 'Ǽ'=>'A',
        'à'=>'a', 'á'=>'a', 'â'=>'a', 'ã'=>'a', 'ä'=>'a', 'å'=>'a', 'ă'=>'a', 'ā'=>'a', 'ą'=>'a', 'æ'=>'a', 'ǽ'=>'a',

        'Þ'=>'B', 'þ'=>'b', 'ß'=>'Ss',

        'Ç'=>'C', 'Č'=>'C', 'Ć'=>'C', 'Ĉ'=>'C', 'Ċ'=>'C',
        'ç'=>'c', 'č'=>'c', 'ć'=>'c', 'ĉ'=>'c', 'ċ'=>'c',

        'Đ'=>'Dj', 'Ď'=>'D',
        'đ'=>'dj', 'ď'=>'d',

        'È'=>'E', 'É'=>'E', 'Ê'=>'E', 'Ë'=>'E', 'Ĕ'=>'E', 'Ē'=>'E', 'Ę'=>'E', 'Ė'=>'E',
        'è'=>'e', 'é'=>'e', 'ê'=>'e', 'ë'=>'e', 'ĕ'=>'e', 'ē'=>'e', 'ę'=>'e', 'ė'=>'e',

        'Ĝ'=>'G', 'Ğ'=>'G', 'Ġ'=>'G', 'Ģ'=>'G',
        'ĝ'=>'g', 'ğ'=>'g', 'ġ'=>'g', 'ģ'=>'g',

        'Ĥ'=>'H', 'Ħ'=>'H',
        'ĥ'=>'h', 'ħ'=>'h',

        'Ì'=>'I', 'Í'=>'I', 'Î'=>'I', 'Ï'=>'I', 'İ'=>'I', 'Ĩ'=>'I', 'Ī'=>'I', 'Ĭ'=>'I', 'Į'=>'I',
        'ì'=>'i', 'í'=>'i', 'î'=>'i', 'ï'=>'i', 'į'=>'i', 'ĩ'=>'i', 'ī'=>'i', 'ĭ'=>'i', 'ı'=>'i',

        'Ĵ'=>'J',
        'ĵ'=>'j',

        'Ķ'=>'K',
        'ķ'=>'k', 'ĸ'=>'k',

        'Ĺ'=>'L', 'Ļ'=>'L', 'Ľ'=>'L', 'Ŀ'=>'L', 'Ł'=>'L',
        'ĺ'=>'l', 'ļ'=>'l', 'ľ'=>'l', 'ŀ'=>'l', 'ł'=>'l',

        'Ñ'=>'N', 'Ń'=>'N', 'Ň'=>'N', 'Ņ'=>'N', 'Ŋ'=>'N',
        'ñ'=>'n', 'ń'=>'n', 'ň'=>'n', 'ņ'=>'n', 'ŋ'=>'n', 'ʼn'=>'n',

        'Ò'=>'O', 'Ó'=>'O', 'Ô'=>'O', 'Õ'=>'O', 'Ö'=>'O', 'Ø'=>'O', 'Ō'=>'O', 'Ŏ'=>'O', 'Ő'=>'O', 'Œ'=>'O',
        'ò'=>'o', 'ó'=>'o', 'ô'=>'o', 'õ'=>'o', 'ö'=>'o', 'ø'=>'o', 'ō'=>'o', 'ŏ'=>'o', 'ő'=>'o', 'œ'=>'o', 'ð'=>'o',

        'Ŕ'=>'R', 'Ř'=>'R',
        'ŕ'=>'r', 'ř'=>'r', 'ŗ'=>'r',

        'Š'=>'S', 'Ŝ'=>'S', 'Ś'=>'S', 'Ş'=>'S',
        'š'=>'s', 'ŝ'=>'s', 'ś'=>'s', 'ş'=>'s',

        'Ŧ'=>'T', 'Ţ'=>'T', 'Ť'=>'T',
        'ŧ'=>'t', 'ţ'=>'t', 'ť'=>'t',

        'Ù'=>'U', 'Ú'=>'U', 'Û'=>'U', 'Ü'=>'U', 'Ũ'=>'U', 'Ū'=>'U', 'Ŭ'=>'U', 'Ů'=>'U', 'Ű'=>'U', 'Ų'=>'U',
        'ù'=>'u', 'ú'=>'u', 'û'=>'u', 'ü'=>'u', 'ũ'=>'u', 'ū'=>'u', 'ŭ'=>'u', 'ů'=>'u', 'ű'=>'u', 'ų'=>'u',

        'Ŵ'=>'W', 'Ẁ'=>'W', 'Ẃ'=>'W', 'Ẅ'=>'W',
        'ŵ'=>'w', 'ẁ'=>'w', 'ẃ'=>'w', 'ẅ'=>'w',

        'Ý'=>'Y', 'Ÿ'=>'Y', 'Ŷ'=>'Y',
        'ý'=>'y', 'ÿ'=>'y', 'ŷ'=>'y',

        'Ž'=>'Z', 'Ź'=>'Z', 'Ż'=>'Z',
        'ž'=>'z', 'ź'=>'z', 'ż'=>'z',

        '“'=>'"', '”'=>'"', '‘'=>"'", '’'=>"'", '•'=>'-', '…'=>'...', '—'=>'-', '–'=>'-', '¿'=>'?', '¡'=>'!', '°'=>' degrees ',
        '¼'=>' 1/4 ', '½'=>' 1/2 ', '¾'=>' 3/4 ', '⅓'=>' 1/3 ', '⅔'=>' 2/3 ', '⅛'=>' 1/8 ', '⅜'=>' 3/8 ', '⅝'=>' 5/8 ', '⅞'=>' 7/8 ',
        '÷'=>' divided by ', '×'=>' times ', '±'=>' plus-minus ', '√'=>' square root ', '∞'=>' infinity ',
        '≈'=>' almost equal to ', '≠'=>' not equal to ', '≡'=>' identical to ', '≤'=>' less than or equal to ', '≥'=>' greater than or equal to ',
        '←'=>' left ', '→'=>' right ', '↑'=>' up ', '↓'=>' down ', '↔'=>' left and right ', '↕'=>' up and down ',
        '℅'=>' care of ', '℮' => ' estimated ',
        'Ω'=>' ohm ',
        '♀'=>' female ', '♂'=>' male ',
        '©'=>' Copyright ', '®'=>' Registered ', '™' =>' Trademark ',
    );

    $string = strtr($string, $table);
    // Currency symbols: £¤¥€  - we dont bother with them for now
    $string = preg_replace("/[^\x9\xA\xD\x20-\x7F]/u", "", $string);

    return $string;
}

The easiest way to do that would be

echo transliterator_transliterate('Any-Latin; Latin-ASCII', "Höhle"); // returns Hohle