- array_diff_assoc: documentation ( source)
- var_dump: documentation ( source)
- array_unique: documentation ( source)
- str_split: documentation ( source)
<?php
$alpha = 'aabbccddeeffgghhiijjkk';
$alphaArray = str_split($alpha);
$uniqArray = array_unique($alphaArray);
$diff = array_diff_assoc($alphaArray, $uniqArray);
var_dump($diff);