ctype_alnum

<?php         $encoded = '';

        for ($i = 0; isset($string[$i]); $i++) {

            $character = $string[$i];

            if (!ctype_alnum($character) && !in_array($character, array('-', '_'), true)) {

                $character = '%' . sprintf('%02X', ord($character));

            }

            $encoded .= $character;