php: number only hash?

Question

In php is there a way to give a unique hash from a string, but that the hash was made up from numbers only?

example:

return md5(234); // returns 098f6bcd4621d373cade4e832627b4f6

but I need

return numhash(234); // returns 00978902923102372190 
(20 numbers only)

the problem here is that I want the hashing to be short.

edit: OK let me explain the back story here. I have a site that has a ID for every registered person, also I need a ID for the person to use and exchange (hence it can't be too long), so far the ID numbering has been 00001, 00002, 00003 etc...

1. this makes some people look more important
2. this reveals application info that I don't want to reveal.

To fix point 1 and 2 I need to "hide" the number while keeping it unique.

Edit + SOLUTION:

Numeric hash function based on the code by https://stackoverflow.com/a/23679870/175071

/**
 * Return a number only hash
 * https://stackoverflow.com/a/23679870/175071
 * @param $str
 * @param null $len
 * @return number
 */
public function numHash($str, $len=null)
{
    $binhash = md5($str, true);
    $numhash = unpack('N2', $binhash);
    $hash = $numhash[1] . $numhash[2];
    if($len && is_int($len)) {
        $hash = substr($hash, 0, $len);
    }
    return $hash;
}

// Usage
numHash(234, 20); // always returns 6814430791721596451

Answer 1

An MD5 or SHA1 hash in PHP returns a hexadecimal number, so all you need to do is convert bases. PHP has a function that can do this for you:

$bignum = hexdec( md5("test") );

or

$bignum = hexdec( sha1("test") );

PHP Manual for hexdec

Since you want a limited size number, you could then use modular division to put it in a range you want.

$smallnum = $bignum % [put your upper bound here]

EDIT

As noted by Artefacto in the comments, using this approach will result in a number beyond the maximum size of an Integer in PHP, and the result after modular division will always be 0. However, taking a substring of the hash that contains the first 16 characters doesn't have this problem. Revised version for calculating the initial large number:

$bignum = hexdec( substr(sha1("test"), 0, 15) );