We have an array with many objects in a style like
<?php
$step = new StepModel(1); // if the StepModel id is "1"
$demand = $step->getDemand(); // returns DemandModel
$step2 = $demand->getCustomer(); // returns StepModel
$demand2 = $step2->getDemand(); // returns DemandModel
// [ ... ]
?>

$step and $step2 can be the same objects. So we have an recursive array. Now we need to know if $step == $step2 or $step === $step2. In other words: We need to know the php internal resource ids.
Because there is no function in php api, we made the following function.
Be careful: In our case, all objects have as first attribute ["id":protected]. If your objects are different from this, you need to edit $pattern.
Warning: function is very slow and should only be called if it's necessary for debugging reasons:
<?php
/**
 * returns resource- and object-ids of all objects in an array
 * 
 * @param array $array
 * @return array
 */
function getObjectInformation(Array $array)
{
 // start output-buffering
 ob_start();
 
 // create an var_dump of $array
 var_dump($array);
 
 // save the dump in var $dump
 $dump = ob_get_contents();
 
 // clean the output-buffer
 ob_end_clean();
 
 // delete white-spaces
 $dump = str_replace(' ', '', $dump);
 
 // define the regex-pattern
 // in our case, all objects look like this:
 //
 // object(ClassName)#1(1){
 // ["id":protected]=>
 // string(1)"1"
 $pattern = '/object\(([a-zA-Z0-9]+)\)#([0-9]+)\([0-9]+\){\\n';
 $pattern .= '\["id":protected\]=>\\n';
 $pattern .= 'string\([0-9]+\)"([v]?[0-9]+)"/im';
 
 // search for all matches
 preg_match_all($pattern, $dump, $regs);
 
 // sort all mathes by class name, object id and then resource id
 array_multisort($regs[1], SORT_ASC, SORT_STRING,
 $regs[3], SORT_ASC, SORT_NUMERIC,
 $regs[2], SORT_ASC, SORT_NUMERIC);
 
 // cache the last match
 $lastMatch = array();
 
 // the return value
 $return = array();
 
 // loop through the matches
 for ($i = 0; $i < sizeof($regs[0]); $i ++) {
 
 // check if the current match was not visited before
 if (0 == sizeof($lastMatch) ||
 $regs[1][$i] != $lastMatch[1] ||
 $regs[2][$i] != $lastMatch[2] ||
 $regs[3][$i] != $lastMatch[3]) {
 
 // save the match in return value
 array_push($return, array($regs[1][$i],
 $regs[2][$i],
 $regs[3][$i]));
 }
 
 // save match in last match cache
 $lastMatch[1] = $regs[1][$i];
 $lastMatch[2] = $regs[2][$i];
 $lastMatch[3] = $regs[3][$i];
 }
 
 // return all matches
 return $return;
}
?>

I know, it's not that elegant but I hope it's useful for a few people.