<?php
function hashPbkdf2($algorithm, $password, $salt, $iterations, $length = 0)
{
    // Number of blocks needed to create the derived key
    $blocks = ceil($length / strlen(hash($algorithm, null, true)));
    $digest = '';
    $length = strlen(hash($algorithm, '', true));
    if (strlen($password) > $length) {
        $password = hash($algorithm, $password, true);
    }

    for ($i = 1; $i <= $blocks; $i++) {
        $ib = $block = hash_hmac($algorithm, $salt . pack('N', $i), $password, true);
        // Iterations
        for ($j = 1; $j < $iterations; $j++) {
            $ib ^= ($block = hash_hmac($algorithm, $block, $password, true));
        }
        $digest .= $ib;
    }
    return substr($digest, 0, 40);
}
function hashPbkdf2Original($algorithm, $password, $salt, $iterations, $length = 0)
{
    // Number of blocks needed to create the derived key
    $blocks = ceil($length / strlen(hash($algorithm, null, true)));
    $digest = '';
    for ($i = 1; $i <= $blocks; $i++) {
        $ib = $block = hash_hmac($algorithm, $salt . pack('N', $i), $password, true);
        // Iterations
        for ($j = 1; $j < $iterations; $j++) {
            $ib ^= ($block = hash_hmac($algorithm, $block, $password, true));
        }
        $digest .= $ib;
    }
    return substr($digest, 0, 40);
}

function bench($p, $s, $i)
{
	echo 'pw length: ' . strlen($p) . "\n";
	echo 'iterations: ' . $i . "\n";

	$start = microtime(true);
	$h1 = bin2hex(hashPbkdf2('sha256', $p, $s, $i, 32));
	echo 'polyfill: ' . (microtime(true) - $start) . "s\n";

	$start = microtime(true);
	$h2 = hash_pbkdf2('sha256', $p, $s, $i);
	echo 'native: ' . (microtime(true) - $start) . "s\n";

	echo 'h1 === h2: ' . ($h1 === $h2 ? 'true' : 'false') . "\n\n";
}

$iter = 1e4;
bench(str_repeat('*', 1e2), 'somerandombytes', $iter);
bench(str_repeat('*', 1e3), 'somerandombytes', $iter);
bench(str_repeat('*', 1e4), 'somerandombytes', $iter);
bench(str_repeat('*', 1e5), 'somerandombytes', $iter);