[man]urlencode/man can be used to change special characters to it's entities that will be used when transporting a value in the URL. [man]urldecode/man does the opposite, it transforms entities into it's real characters.
<?php
// some text using swedish chars
$text = 'hej på dig, göran';
// encode the string
$encoded = urlencode($text).'<br/>';
echo $encoded.'<br/>';
// decode the string
echo urldecod($encoded);
?>
