spl_object_hash for PHP < 5.2 (unique ID for object instances)

Question

I'm trying to get unique IDs for object instances in PHP 5+.

The function, spl_object_hash() is available from PHP 5.2 but I'm wondering if there is a workaround for older PHP versions.

There are a couple of functions in the comments on php.net but they're not working for me. The first (simplified):

function spl_object_hash($object){
    if (is_object($object)){
        return md5((string)$object);
        }
    return null;
    }

does not work with native objects (such as DOMDocument), and the second:

function spl_object_hash($object){
    if (is_object($object)){
        ob_start();
        var_dump($object);
        $dump = ob_get_contents();
        ob_end_clean();
        if (preg_match('/^object\(([a-z0-9_]+)\)\#(\d)+/i', $dump, $match)) {
            return md5($match[1] . $match[2]);
            }
        }
    return null;
    }

looks like it could be a major performance buster!

Does anybody have anything up their sleeve?

Answer 1

I ran a couple of quick tests. I really think you'd be better off storing real callbacks in your bind() function using bind('evt_name', array($obj, 'callback_function')). If you absolutely want to go the spl_object_hash route, rather than storing references with the event bindings, you're looking at something like this:

A var_dump / extract and hash id implementation:

function spl_object_hash_var_dump($object){
    if (is_object($object)){
        ob_start();
        var_dump($object);
        $dump = ob_get_contents();
        ob_end_clean();
        if (preg_match('/^object\(([a-z0-9_]+)\)\#(\d)+/i', $dump, $match)) {
            return md5($match[1] . $match[2]);
            }
        }
    return null;
}

A naive references implementation:

function spl_object_dumb_references(&$object) {
    static $hashes;

    if (!isset($hashes)) $hashes = array();

    // find existing instance
    foreach ($hashes as $hash => $o) {
        if ($object === $o) return $hash;
    }

    $hash = md5(uniqid());
    while (array_key_exists($hash, $hashes)) {
        $hash = md5(uniqid());
    }

    $hashes[$hash] = $object;
    return $hash;
}

This one was basically 5-50x worse than the class-based reference function across the board, so it's not worth worrying about.

A store references by class implementation:

function spl_object_hash_references(&$object) {
    static $hashes;

    if (!isset($hashes)) $hashes = array();

    $class_name = get_class($object);
    if (!array_key_exists($class_name, $hashes)) {
        $hashes[$class_name] = array();
    }

    // find existing instance
    foreach ($hashes[$class_name] as $hash => $o) {
        if ($object === $o) return $hash;
    }

    $hash = md5(uniqid($class_name));
    while (array_key_exists($hash, $hashes[$class_name])) {
        $hash = md5(uniqid($class_name));
    }

    $hashes[$class_name][$hash] = $object;
    return $hash;
}

And you end up with results that look like this. Summary: the class based references implementation performs best around n/50 classes--at its best, it manages to pull off 1/3 the performance of the var_dump based implementation, and it's usually much worse.

The var_dump implementation seems to be tolerable, though not ideal. But if you're not doing too many of these lookups, it won't be a bottleneck for you. Especially as a fallback for PHP < 5.2 boxen.

Answer 2

I once wrote a helper function for wordpress that offers one unique hash per object, it works with a counter and stores the hash per as a public class property if it has been assigned to an object. The following example demonstrates this:

/**
 * get object hash
 *
 * Returns a unique hash per object.
 *
 * Proxy function for wordpress installments on servers
 * with a PHP version < 5.2.0.
 *
 * @since 3.0.2
 * @note Become deprecated with version 3.2.0 (PHP 5.2 requirements)
 * @param object $object
 * @return string unique object hash
 */
function wp_object_hash( &$object ) {
    static $prefix, $count = 0, $property = '__wphookobjhash__', $spl_function_exists;

    isset( $spl_function_exists ) || $spl_function_exists = function_exists( 'spl_object_hash' );

    // prefer spl_object_hash if available
    if ( $spl_function_exists )
        return spl_object_hash( $object );

    // validate input
    if ( !is_object( $object ) ) { 
        trigger_error( __FUNCTION__ . '() expects parameter 1 to be object', E_USER_WARNING );
        return null;
    }
    // setup prefix and counter to generate object hash, set it to object if not set
    isset( $prefix ) || ( ( $prefix = uniqid( '' ) ) && $property .= $prefix . '__' );
    isset( $object->$property ) || ( $object->$property = sprintf( '%s-%08d', $prefix , ++$count ) );
    return $object->$property;
}

If you're using a PHP 5 version, you don't need to pass the parameter by reference.

Answer 3

This is what you want.

I've fixed a very probable bug and streamlined the function from bobthecow answer (which is also borrowed from php.net) to this:

if ( !function_exists( 'spl_object_hash' ) ) {
    function spl_object_hash( $object )
    {
        ob_start();
        var_dump( $object );
        preg_match( '[#(\d+)]', ob_get_clean(), $match );
        return $match[1];
    }
}

It returns an integer (usually in the sub-100 range), which is unique for any object (see this answer for details on what you're seeing).

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P.S. I use this implementation in a real world scenario here