$array = [2,5,7,9,11];
$target = 11;

function twoSums($array, $target) {
    $count = count($array);
    for ($i=0;$i<$count;$i++) {
        $diff = $target - $array[$i];
        $newArray = $array;
        unset($newArray[$i]);

        if (in_array($diff, $newArray)) {
            return [$i, array_search($diff, $newArray)];
        }
    }
}

$a=twoSums($array, $target);
var_dump($a);