Quick check of the table at http://en.wikipedia.org/wiki/Birthday_attack

birthday.php

<?php

$hashes = [];
$count = 0;
while (true) {
    $randBytes = openssl_random_pseudo_bytes(100);
    ++$count;
    $hash = hash('md5', $randBytes);
    $smallerHash = substr($hash, 0, 8);
    if (isset($hashes[$smallerHash])) {
        $collision = base64_encode($hashes[$smallerHash]);
        printf("Collision found after %s, %s and %s both hash to %s", $count, $collision, base64_encode($randBytes), $smallerHash);
        echo PHP_EOL;
        break;
    }
    $hashes[$smallerHash] = $randBytes;
}