<?php

$a1 = array("one","two","three");
$found = "two";
$notFound = "four";

var_dump(in_array($found,$a1));echo "\n";
var_dump(in_array($notFound,$a1));echo "\n";

var_dump(count(array_diff($a1,array($found))) != count($a1));echo "\n";
var_dump(count(array_diff($a1,array($notFound))) != count($a1));

$found = array("one","three");

var_dump(count(array_diff($a1,$found)) != count($a1));echo "\n";

bool(true)

bool(false)

bool(true)

bool(false)
bool(true)