<?php
function two($a, $t) {
    $b = $a;
    foreach($a as $k => $v) {
        unset($b[$k]);
        foreach($b as $bk => $bv) {
            if($v + $bv == $t) {
                return [$k, $bk];
            }
        }
    }
    return [];
}

print_r(two([1, -4, 6, 9, 4, 3, 8], 5));
?>