map English alphabets to their position in qwerty layout keyboard, and reverse

qwerty.php

<?php

/*
 * QWERTY encode decode
 *
 * qwerty_encode maps English alphabets to their
 * position in qwerty layout keyboard like so:
 *   a -> q, b -> w, c -> e ... etc
 *
 * qwerty_decode does the reverse
 */

function qwerty_map()
{
    $q = 'qwertyuiopasdfghjklzxcvbnm';

    return array_combine(
        array_merge(range('a', 'z'), range('A', 'Z')),
        str_split($q.strtoupper($q))
    );
}

function qwerty_encode($str)
{
    if (!is_string($str) or !preg_match('/[a-z]/i', $str)) {
        return $str;
    }
    $ret = '';
    $map = qwerty_map();
    foreach (str_split($str) as $chr) {
        $ret .= isset($map[$chr]) ? $map[$chr] : $chr;
    }

    return $ret;
}

function qwerty_decode($str)
{
    if (!is_string($str) or !preg_match('/[a-z]/i', $str)) {
        return $str;
    }
    $ret = '';
    $map = array_flip(qwerty_map());
    foreach (str_split($str) as $chr) {
        $ret .= isset($map[$chr]) ? $map[$chr] : $chr;
    }

    return $ret;
}