Detecting whether a PHP variable is a reference / referenced

Question

Is there a way in PHP to determine whether a given variable is a reference to another variable and / or referenced by another variable? I appreciate that it might not be po

Answer 1

Take a peak at xdebug_debug_zval(). Right now, that's the only way to really know if you can determine everything about the variable's zval.

So here are a couple of helper functions to determine some helpful information:

function isRef($var) {
    $info = getZvalRefCountInfo($var);
    return (boolean) $info['is_ref'];
}
function getRefCount($var) {
    $info = getZvalRefCountInfo($var);
    return $info['refcount'];
}
function canCopyOnWrite($var) {
    $info = getZvalRefCountInfo($var);
    return $info['is_ref'] == 0;
}
function canReferenceWithoutCopy($var) {
    $info = getZvalRefCountInfo($var);
    return $info['is_ref'] == 1 || $info['refcount'] == 1;
}

function getZvalRefCountInfo($var) {
    ob_start();
    xdebug_debug_zval($var);
    $info = ob_get_clean();
    preg_match('(: \(refcount=(\d+), is_ref=(\d+)\))', $info, $match);
    return array('refcount' => $match[1], 'is_ref' => $match[2]);
}

So with some sample variables:

$a = 'test';
$b = $a;
$c = $b;
$d =& $c;
$e = 'foo';

We can test if a variable is a reference:

isRef('a'); // false
isRef('c'); // true
isRef('e'); // false

We can get the number of variables linked to the zval (not necessarily a reference, can be for copy-on-write):

getRefCount('a'); // 2
getRefCount('c'); // 2
getRefCount('e'); // 1

We can test if we can copy-on-write (copy without performing a memory copy):

canCopyOnWrite('a'); // true
canCopyOnWrite('c'); // false
canCopyOnWrite('e'); // true

And we can test if we can make a reference without copying the zval:

canReferenceWithoutCopy('a'); // false
canReferenceWithoutCopy('c'); // true
canReferenceWithoutCopy('e'); // true

And now, we can check if a variable references itself through some black magic:

function isReferenceOf(&$a, &$b) {
    if (!isRef('a') || getZvalRefCountInfo('a') != getZvalRefCountInfo('b')) {
        return false;
    }
    $tmp = $a;
    if (is_object($a) || is_array($a)) {
        $a = 'test';
        $ret = $b === 'test';
        $a = $tmp;
    } else {
        $a = array();
        $ret = $b === array();
        $a = $tmp;
    }
    return $tmp;
}

It's a bit hacky since we can't determine what other symbols reference the same zval (only that other symbols reference). So this basically checks to see if $a is a reference, and if $a and $b both have the same refcount and reference flag set. Then, it changes one to check if the other changes (indicating they are the same reference).

Answer 2

Full working example:

function EqualReferences(&$first, &$second){
    if($first !== $second){
        return false;
    } 
    $value_of_first = $first;
    $first = ($first === true) ? false : true; // modify $first
    $is_ref = ($first === $second); // after modifying $first, $second will not be equal to $first, unless $second and $first points to the same variable.
    $first = $value_of_first; // unmodify $first
    return $is_ref;
}

$a = array('foo');
$b = array('foo');
$c = &$a;
$d = $a;

var_dump(EqualReferences($a, $b)); // false
var_dump(EqualReferences($b, $c)); // false
var_dump(EqualReferences($a, $c)); // true
var_dump(EqualReferences($a, $d)); // false
var_dump($a); // unmodified
var_dump($b); // unmodified

Answer 3

Maybe xdebug_debug_zval() helps you. http://www.xdebug.org/docs/all_functions

Answer 4

You can use debug_zval_dump:

function countRefs(&$var) {
    ob_start();
    debug_zval_dump(&$var);
    preg_match('~refcount\((\d+)\)~', ob_get_clean(), $matches);
    return $matches[1] - 4;
}

$var = 'A';
echo countRefs($var); // 0

$ref =& $var;
echo countRefs($var); // 1

This though will not work anymore as of PHP 5.4 as they removed call time pass by reference support and may throw an E_STRICT level error on lower versions.

If you wonder, where the -4 in the above function come from: You tell me... I got it by trying. In my eyes it should be only 3 (the variable, the variable in my function, the variable passed to zend_debug_zval), but I'm not too good at PHP internals and it seems that it creates yet another reference somewhere on the way ;)

Answer 5

Edit: It seems I've answered the question 'is it possible to check if two variables are referencing same value in memory' not the actual question asked. :P

---

As far as 'plain' variables go the answer is 'no'.

As far as objects go - maybe.

All objects are by default handled by references. Also each object has it's serial number which you can see when you var_dump() it.

>> class a {};
>> $a = new a();
>> var_dump($a);

object(a)#12 (0) {
}

If you could get somehow to this #, you could effectively compare it for two variables, and see if they point to the same object. The question is how to get this number. var_export() does not return it. I don't see snything in Reflection classes that would get it either.

One thing that comes to my mind is using output buffering + regex