I found that this approach is often faster than strtr() and won't change the same thing in your string twice (as opposed to str_replace(), which will overwrite things in the order of the array you feed it with):

<?php
function replace ($text, $replace) {
    $keys = array_keys($replace);
    $length = array_combine($keys, array_map('strlen', $keys));
    arsort($length);
    
    $array[] = $text;
    $count = 1;
    reset($length);
    while ($key = key($length)) {
        if (strpos($text, $key) !== false) {
            for ($i = 0; $i < $count; $i += 2) {
                if (($pos = strpos($array[$i], $key)) === false) continue;
                array_splice($array, $i, 1, array(substr($array[$i], 0, $pos), $replace[$key], substr($array[$i], $pos + strlen($key))));
                $count += 2;
            }
        }
        next($length);
    }
    return implode($array);
}
?>