Represent MD5 hash as an integer

Be careful. Converting the MD5s to an integer will require support for big (128-bit) integers. Chances are the API you're using will only support 32-bit integers - or worse, might be dealing with the number in floating-point. Either way, your ID will get munged. If this is the case, just assigning a second ID arbitrarily is a much better way to deal with things than trying to convert the MD5 into an integer.

However, if you are sure that the API can deal with arbitrarily large integers without trouble, you can just convert the MD5 from hexadecimal to an integer. PHP most likely does not support this built-in however, as it will try to represent it as either a 32-bit integer or a floating point; you'll probably need to use the PHP GMP library for it.

There are good reasons, stated by others, for doing it a different way.

But if what you want to do is convert an md5 hash into a string of decimal digits (which is what I think you really mean by "represent by an integer", since an md5 is already an integer in string form), and transform it back into the same md5 string:

function md5_hex_to_dec($hex_str)
{
    $arr = str_split($hex_str, 4);
    foreach ($arr as $grp) {
        $dec[] = str_pad(hexdec($grp), 5, '0', STR_PAD_LEFT);
    }
    return implode('', $dec);
}

function md5_dec_to_hex($dec_str)
{
    $arr = str_split($dec_str, 5);
    foreach ($arr as $grp) {
        $hex[] = str_pad(dechex($grp), 4, '0', STR_PAD_LEFT);
    }
    return implode('', $hex);
}

Demo:

$md5 = md5('[email protected]');
echo $md5 . '<br />';  // 23463b99b62a72f26ed677cc556c44e8
$dec = md5_hex_to_dec($md5);
echo $dec . '<br />';  // 0903015257466342942628374306682186817640
$hex = md5_dec_to_hex($dec);
echo $hex;             // 23463b99b62a72f26ed677cc556c44e8

Of course, you'd have to be careful using either string, like making sure to use them only as string type to avoid losing leading zeros, ensuring the strings are the correct lengths, etc.