test.php

<?php

function my_array_diff($a, $b)
{
    $map = $out = array();
    foreach ($a as $val)
        $map[$val] = 1;
    foreach ($b as $val)
        if (isset($map[$val]))
            $map[$val] = 0;
    foreach ($map as $val => $ok)
        if ($ok)
            $out[]     = $val;
    return $out;
}

function your_array_diff($arraya, $arrayb)
{
    foreach ($arraya as $keya => $valuea) {
        if (in_array($valuea, $arrayb)) {
            unset($arraya[$keya]);
        }
    }
    return $arraya;
}

function leo_array_diff($a, $b)
{
    $map = $out  = array();
    foreach ($a as $val)
        $map[$val] = 1;
    foreach ($b as $val)
        unset($map[$val]);
    return array_keys($map);
}

function flip_array_diff_key($b, $a)
{
    $at = array_flip($a);
    $bt = array_flip($b);
    $d  = array_diff_key($bt, $at);
    return array_keys($d);
}

function flip_isset_diff($b, $a)
{
    $at  = array_flip($a);
    $d   = array();
    foreach ($b as $i)
        if (!isset($at[$i]))
            $d[] = $i;
    return $d;
}

function large_array_diff($b, $a)
{
    $at     = array();
    foreach ($a as $i)
        $at[$i] = 1;

    $d = array();

    foreach ($b as $i)
        if (!isset($at[$i]))
            $d[] = $i;

    return $d;
}

$a = range(1, 20000);
$b = range(5000, 25000);

shuffle($a);
shuffle($b);


$ts = microtime(true);
array_diff($a, $b);
printf("PHP =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
my_array_diff($a, $b);
printf("my_array_diff =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
your_array_diff($a, $b);
printf("your_array_diff =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
leo_array_diff($a, $b);
printf("leo_array_diff =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
flip_array_diff_key($a, $b);
printf("flip_array_diff_key =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
flip_isset_diff($a, $b);
printf("flip_isset_diff =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
large_array_diff($a, $b);
printf("large_array_diff =%.4f\n", microtime(true) - $ts);