I need a unique string from an array so that I can tell when it changes without measuring the inputs of that array. I'm trying to work out if it is computationally efficient to calculate a value rather than add code to look out for changes in the array. The array itself can have a variety of values and for future proofing I don't want to try and measure whether new values have been added to the array, I'd much rather just create some string or hash that will change if the array itself changes.

So for example:

$a = Array(
'var1' => 1,
'var2' => 2,
'var3' => 3,

If I was to use md5(http_build_query($a)) perhaps with an added ksort to confirm that the order of the keys haven't changed this might then produce a unique string that I can use to compare against another run of the application to evaluate whether the array has changed.

I'm looking for an alternate, possibly faster or more elegant solutions to this.


array_diff() ?

Written by jnpcl

I don't think array_diff would deal with checking if the key order has changed. You could json_encode the array and take a hash of that. Might want to check the performance of json_encode and http_build_query.

Written by Endophage


Written by yes123

Accepted Answer

Thanks for all the ideas guys.

I've tried all of them except a sha-256 which my server doesn't have installed.

Here's the results:

Average (http_build_query): 1.3954045954045E-5
Average (diff): 0.00011533766233766
Average (serialize): 1.7588411588412E-5
Average (md5): 1.6036963036966E-5
Average (implode-haval160,4): 1.5349650349649E-5

That's running the operation 1000 times and averaging the result. After refreshing a couple times I could tell that the http_build_query was the quickest. I guess my next question would be if anyone can think of any pitfalls of using this method?


Here's my code:

class a {

    static $input;

    function test() {
        $start = null;
        $s = $e = $d = $g = $h = $i = $k = array();
        self::$input = array();

        for ($x = 0; $x <= 30; $x++) {
            self::$input['variable_' . $x] = rand();

        for ($x = 0; $x <= 1000; $x++) {
            $start = microtime();

            $c = http_build_query(self::$input);
            ($c == $c);

            $s[] = microtime() - $start;

        for ($x = 0; $x <= 1000; $x++) {
            $start = microtime();

            $c = md5(http_build_query(self::$input));
            ($c == $c);

            $e[] = microtime() - $start;

        for ($x = 0; $x <= 1000; $x++) {
            $start = microtime();

            $c = array_diff(self::$input, self::$input);

            $d[] = microtime() - $start;
        for ($x = 0; $x <= 1000; $x++) {
            $start = microtime();

            $c = serialize(self::$input);
            ($c == $c);

            $g[] = microtime() - $start;

        for ($x = 0; $x <= 1000; $x++) {
            $start = microtime();

            $c =  hash("haval160,4", implode(',',self::$input));
            ($c == $c);

            $h[] = microtime() - $start;
        echo "<pre>";

        echo "Average (http_build_query): " . array_sum($s) / count($s) . "<br>";
        echo "Average (diff): " . array_sum($d) / count($d) . "<br>";
        echo "Average (serialize): " . array_sum($g) / count($g) . "<br>";
        echo "Average (md5): " . array_sum($e) / count($e). "<br>";
        echo "Average (implode-haval160,4): " . array_sum($h) / count($h);


Written by Jason
This page was build to provide you fast access to the question and the direct accepted answer.
The content is written by members of the stackoverflow.com community.
It is licensed under cc-wiki