ReenExe

$start = microtime(true);

$exists = array_fill(2e7, 7e7, true);
$stored = array_fill(3e7, 8e7, true);
shuffle($exists);
shuffle($stored);

echo microtime(true) - $start;
echo PHP_EOL;

$start = microtime(true);;

$diff = array_diff($stored, $exists);

echo microtime(true) - $start;
echo PHP_EOL;

$start = microtime(true);

$diff = array_keys(array_diff_key(array_fill_keys($stored, true), array_fill_keys($exists, true)));

echo microtime(true) - $start;
echo PHP_EOL;

// initialize:      11.500846147537
// array_diff:      4.1464519500732
// array_diff_key:  3.2659499645233