array_udiff_wtf.php

<?php
$array1 = array(
    array('A', 1),
    array('B', 1),
);

$array2 = array(
    array('A', 2),
    array('C', 2),
);

// http://devdocs.io/php/function.array-udiff
// Returns an array containing all the values of array1 that are not
// present in any of the other arguments.
$DIFF = array_udiff(
    $array1,
    $array2,
    function ($a, $b) {
        // The comparison function must return an integer less than,
        // equal to, or greater than zero if the first argument is
        // considered to be respectively less than, equal to, or greater
        // than the second.
        return $a[0] === $b[0] ? 0 : 1;
    }
);

var_dump($DIFF); // Would expect to get array(array('A', 1))