Change foreign characters to their roman equivalent

My first recommendation is the iconv function. Namely because it's built into PHP, so doesn't require any external or 3rd party libraries. In addition, it's a function that's designed to do precisely what you are trying to accomplish (accept on character set as input, and output an alternate character set, specifically going from UTF-8 to ASCII). Below is an example of how to call this function:

$clean_ascii_output = iconv('UTF-8', 'ASCII//TRANSLIT', $utf8_input);

More information about the specifics of this PHP function can be found here: http://php.net/manual/en/function.iconv.php

Note: The iconv function accepts string inputs, so you'll want to iterate over data, and parse it such that you are passing in a string input.

I coded this function which uses the HTML entities translation table built-in into PHP to romanize chars:

function Unaccent($string)
{
    if (strpos($string = htmlentities($string, ENT_QUOTES, 'UTF-8'), '&') !== false)
    {
        $string = html_entity_decode(preg_replace('~&([a-z]{1,2})(?:acute|cedil|circ|grave|lig|orn|ring|slash|tilde|uml);~i', '$1', $string), ENT_QUOTES, 'UTF-8');
    }

    return $string;
}

It works by applying htmlentities() and then removing common entities suffixes, a simple example:

 - ã = ã -> a
 - Ã = Ã -> A
 - õ = õ -> o
 - Õ = Õ -> O
 - æ = æ  -> ae
 - Æ = Æ  -> AE

Beware that for this to work properly your files need to be encoded in UTF-8 (no BOM obviously).

See also my other answer for another example.

After failing to find suitable convertors I created my own collection that suits my needs including my favorite Cyrillic conversion that by default has numerous variations.

function transliterateString($txt) {
    $transliterationTable = array('á' => 'a', 'Á' => 'A', 'à' => 'a', 'À' => 'A', 'ă' => 'a', 'Ă' => 'A', 'â' => 'a', 'Â' => 'A', 'å' => 'a', 'Å' => 'A', 'ã' => 'a', 'Ã' => 'A', 'ą' => 'a', 'Ą' => 'A', 'ā' => 'a', 'Ā' => 'A', 'ä' => 'ae', 'Ä' => 'AE', 'æ' => 'ae', 'Æ' => 'AE', 'ḃ' => 'b', 'Ḃ' => 'B', 'ć' => 'c', 'Ć' => 'C', 'ĉ' => 'c', 'Ĉ' => 'C', 'č' => 'c', 'Č' => 'C', 'ċ' => 'c', 'Ċ' => 'C', 'ç' => 'c', 'Ç' => 'C', 'ď' => 'd', 'Ď' => 'D', 'ḋ' => 'd', 'Ḋ' => 'D', 'đ' => 'd', 'Đ' => 'D', 'ð' => 'dh', 'Ð' => 'Dh', 'é' => 'e', 'É' => 'E', 'è' => 'e', 'È' => 'E', 'ĕ' => 'e', 'Ĕ' => 'E', 'ê' => 'e', 'Ê' => 'E', 'ě' => 'e', 'Ě' => 'E', 'ë' => 'e', 'Ë' => 'E', 'ė' => 'e', 'Ė' => 'E', 'ę' => 'e', 'Ę' => 'E', 'ē' => 'e', 'Ē' => 'E', 'ḟ' => 'f', 'Ḟ' => 'F', 'ƒ' => 'f', 'Ƒ' => 'F', 'ğ' => 'g', 'Ğ' => 'G', 'ĝ' => 'g', 'Ĝ' => 'G', 'ġ' => 'g', 'Ġ' => 'G', 'ģ' => 'g', 'Ģ' => 'G', 'ĥ' => 'h', 'Ĥ' => 'H', 'ħ' => 'h', 'Ħ' => 'H', 'í' => 'i', 'Í' => 'I', 'ì' => 'i', 'Ì' => 'I', 'î' => 'i', 'Î' => 'I', 'ï' => 'i', 'Ï' => 'I', 'ĩ' => 'i', 'Ĩ' => 'I', 'į' => 'i', 'Į' => 'I', 'ī' => 'i', 'Ī' => 'I', 'ĵ' => 'j', 'Ĵ' => 'J', 'ķ' => 'k', 'Ķ' => 'K', 'ĺ' => 'l', 'Ĺ' => 'L', 'ľ' => 'l', 'Ľ' => 'L', 'ļ' => 'l', 'Ļ' => 'L', 'ł' => 'l', 'Ł' => 'L', 'ṁ' => 'm', 'Ṁ' => 'M', 'ń' => 'n', 'Ń' => 'N', 'ň' => 'n', 'Ň' => 'N', 'ñ' => 'n', 'Ñ' => 'N', 'ņ' => 'n', 'Ņ' => 'N', 'ó' => 'o', 'Ó' => 'O', 'ò' => 'o', 'Ò' => 'O', 'ô' => 'o', 'Ô' => 'O', 'ő' => 'o', 'Ő' => 'O', 'õ' => 'o', 'Õ' => 'O', 'ø' => 'oe', 'Ø' => 'OE', 'ō' => 'o', 'Ō' => 'O', 'ơ' => 'o', 'Ơ' => 'O', 'ö' => 'oe', 'Ö' => 'OE', 'ṗ' => 'p', 'Ṗ' => 'P', 'ŕ' => 'r', 'Ŕ' => 'R', 'ř' => 'r', 'Ř' => 'R', 'ŗ' => 'r', 'Ŗ' => 'R', 'ś' => 's', 'Ś' => 'S', 'ŝ' => 's', 'Ŝ' => 'S', 'š' => 's', 'Š' => 'S', 'ṡ' => 's', 'Ṡ' => 'S', 'ş' => 's', 'Ş' => 'S', 'ș' => 's', 'Ș' => 'S', 'ß' => 'SS', 'ť' => 't', 'Ť' => 'T', 'ṫ' => 't', 'Ṫ' => 'T', 'ţ' => 't', 'Ţ' => 'T', 'ț' => 't', 'Ț' => 'T', 'ŧ' => 't', 'Ŧ' => 'T', 'ú' => 'u', 'Ú' => 'U', 'ù' => 'u', 'Ù' => 'U', 'ŭ' => 'u', 'Ŭ' => 'U', 'û' => 'u', 'Û' => 'U', 'ů' => 'u', 'Ů' => 'U', 'ű' => 'u', 'Ű' => 'U', 'ũ' => 'u', 'Ũ' => 'U', 'ų' => 'u', 'Ų' => 'U', 'ū' => 'u', 'Ū' => 'U', 'ư' => 'u', 'Ư' => 'U', 'ü' => 'ue', 'Ü' => 'UE', 'ẃ' => 'w', 'Ẃ' => 'W', 'ẁ' => 'w', 'Ẁ' => 'W', 'ŵ' => 'w', 'Ŵ' => 'W', 'ẅ' => 'w', 'Ẅ' => 'W', 'ý' => 'y', 'Ý' => 'Y', 'ỳ' => 'y', 'Ỳ' => 'Y', 'ŷ' => 'y', 'Ŷ' => 'Y', 'ÿ' => 'y', 'Ÿ' => 'Y', 'ź' => 'z', 'Ź' => 'Z', 'ž' => 'z', 'Ž' => 'Z', 'ż' => 'z', 'Ż' => 'Z', 'þ' => 'th', 'Þ' => 'Th', 'µ' => 'u', 'а' => 'a', 'А' => 'a', 'б' => 'b', 'Б' => 'b', 'в' => 'v', 'В' => 'v', 'г' => 'g', 'Г' => 'g', 'д' => 'd', 'Д' => 'd', 'е' => 'e', 'Е' => 'E', 'ё' => 'e', 'Ё' => 'E', 'ж' => 'zh', 'Ж' => 'zh', 'з' => 'z', 'З' => 'z', 'и' => 'i', 'И' => 'i', 'й' => 'j', 'Й' => 'j', 'к' => 'k', 'К' => 'k', 'л' => 'l', 'Л' => 'l', 'м' => 'm', 'М' => 'm', 'н' => 'n', 'Н' => 'n', 'о' => 'o', 'О' => 'o', 'п' => 'p', 'П' => 'p', 'р' => 'r', 'Р' => 'r', 'с' => 's', 'С' => 's', 'т' => 't', 'Т' => 't', 'у' => 'u', 'У' => 'u', 'ф' => 'f', 'Ф' => 'f', 'х' => 'h', 'Х' => 'h', 'ц' => 'c', 'Ц' => 'c', 'ч' => 'ch', 'Ч' => 'ch', 'ш' => 'sh', 'Ш' => 'sh', 'щ' => 'sch', 'Щ' => 'sch', 'ъ' => '', 'Ъ' => '', 'ы' => 'y', 'Ы' => 'y', 'ь' => '', 'Ь' => '', 'э' => 'e', 'Э' => 'e', 'ю' => 'ju', 'Ю' => 'ju', 'я' => 'ja', 'Я' => 'ja');
    return str_replace(array_keys($transliterationTable), array_values($transliterationTable), $txt);
}