What is the most efficient way to remove all the elements of one array from another array?

Question

I have two arrays, array A contains a long list with some elements I want to remove. Array B is the full list of those elements that I wish to remove from array A.

W

Answer 1

array_diff is the obvious answer, but since you've asked for the most efficient way, here's a test

$big = range(1, 90000);
$remove = range(500, 600);

$ts = microtime(true);
$result = array_diff($big, $remove);
printf("%.2f\n", microtime(true) - $ts);

$ts = microtime(true);
$map = array_flip($remove);
$result = array();
foreach($big as $e)
    if(!isset($map[$e]))
        $result[] = $e;
printf("%.2f\n", microtime(true) - $ts);

prints on my machine

0.67
0.03

So the simple loop with a hash-based lookup is approximately 20 times faster than array_diff.