packMD5.php

<?php

// NOTE: you can use actionscript's String.fromCharCode() in place of php's chr()
// NOTE: you can use actionscript's parseInt() in place of php's hexdec()

$md5 = md5("hello"); // php's md5() returns a string
$n = 123456789;

var_dump(pack("H*", $md5) === packMD5($md5)); // output: true
var_dump(pack("VXxx", $n) === packVXxx($n)); // output: true

function packMD5($string) {
	$binary_string = '';
	for ($x = 0; $x < strlen($string); $x += 2) {
		$binary_string .= chr(hexdec($string[$x] . $string[$x + 1]));
	}
	return $binary_string;
}

function packVXxx($n) {
	$binary_string = chr($n & 0x000000ff);
	$binary_string .= chr(($n >> 8) & 0x000000ff);
	$binary_string .= chr(($n >> 16) & 0x000000ff);
	return $binary_string . chr(0) . chr(0);
}