PHP best way to MD5 multi-dimensional array?

Question

What is the best way to generate an MD5 (or any other hash) of a multi-dimensional array?

I could easily write a loop which would traverse through each level of the

Answer 1

(Copy-n-paste-able function at the bottom)

As mentioned prior, the following will work.

md5(serialize($array));

However, it's worth noting that (ironically) json_encode performs noticeably faster:

md5(json_encode($array));

In fact, the speed increase is two-fold here as (1) json_encode alone performs faster than serialize, and (2) json_encode produces a smaller string and therefore less for md5 to handle.

Edit: Here is evidence to support this claim:

';

//The json test
$b4_j = microtime(1);
for ($i=0;$i<10000;$i++) {
    $serial = md5(json_encode($array));
}
echo 'json_encode() w/ md5() took: '.($jTime = microtime(1)-$b4_j).' sec

';
echo 'json_encode is '.( round(($sTime/$jTime)*100,1) ).'% faster with a difference of '.($sTime-$jTime).' seconds';

JSON_ENCODE is consistently over 250% (2.5x) faster (often over 300%) -- this is not a trivial difference. You may see the results of the test with this live script here:

• http://nathanbrauer.com/playground/serialize-vs-json.php
• http://nathanbrauer.com/playground/plain-text/serialize-vs-json.php

Now, one thing to note is array(1,2,3) will produce a different MD5 as array(3,2,1). If this is NOT what you want. Try the following code:

//Optionally make a copy of the array (if you want to preserve the original order)
$original = $array;

array_multisort($array);
$hash = md5(json_encode($array));

Edit: There's been some question as to whether reversing the order would produce the same results. So, I've done that (correctly) here:

• http://nathanbrauer.com/playground/json-vs-serialize.php
• http://nathanbrauer.com/playground/plain-text/json-vs-serialize.php

As you can see, the results are exactly the same. Here's the (corrected) test originally created by someone related to Drupal:

• http://nathanjbrauer.com/playground/drupal-calculation.php
• http://nathanjbrauer.com/playground/plain-text/drupal-calculation.php

And for good measure, here's a function/method you can copy and paste (tested in 5.3.3-1ubuntu9.5):

function array_md5(Array $array) {
    //since we're inside a function (which uses a copied array, not 
    //a referenced array), you shouldn't need to copy the array
    array_multisort($array);
    return md5(json_encode($array));
}