MI
А вообще вот
<?php
$m = [0, 2, 4, 6, 8, 10, 12];
$n = 12;
$checkArr = [];
foreach($m as $key=>$value) {
$searchVal = $n - $value;
if(array_key_exists($searchVal, $checkArr)) {
echo $key."+".$checkArr[$searchVal].PHP_EOL;
}
$checkArr[$value]=$key;
}