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).