Fatal error: Nesting level too deep - recursive dependency?

Question

I have a complex hierarchy of nested objects, with all of the child objects (stored an array of objects in the parent class) containing a property linking back to their parent: fairly simple and straightforward, with no real problems. If I do a var_dump of any object in the hierarchy, I'll get a recursive reference in the dump, exactly as I'd expect.

FIRSTGEN 
   _children array of objects of type SECONDGEN
      SECONDGEN #1
         _parent object of type FIRSTGEN
         _children array of objects of type THIRDGEN
            THIRDGEN #1
               _parent object of type SECONDGEN
            THIRDGEN #2
               _parent object of type SECONDGEN
      SECONDGEN #2
         _parent object of type FIRSTGEN
         _children array of objects of type THIRDGEN
            THIRDGEN #3
               _parent object of type SECONDGEN

I've recently added some new elements to that hierarchy, and they don't follow quite the same pattern. They are stored in an array of objects in the top-level parent, but contain a property linking them back, not to their parent, but to a sibling. When I do a var_dump now, I get a "Fatal error: Nesting level too deep - recursive dependency?".

FIRSTGEN 
   _children_1 array of objects of type SECONDGEN_1
      SECONDGEN_1 #1
         _parent object of type FIRSTGEN
         _children array of objects of type THIRDGEN
            THIRDGEN #1
               _parent object of type SECONDGEN_1
            THIRDGEN #2
               _parent object of type SECONDGEN_1
      SECONDGEN_1 #2
         _parent object of type FIRSTGEN
         _children array of objects of type THIRDGEN
            THIRDGEN #3
               _parent object of type SECONDGEN_1
   _children_2 array of objects of type SECONDGEN_2
      SECONDGEN_2 #1
         _parent object of type SECONDGEN_1

Everything else within the code works correctly, with the exception of that var_dump(). I've tried creating a simpler example to demonstrate the problem, so that I could provide an example when asking this question; but haven't been able to replicate it in a short test, only within my more complex code.

I know that the solution is to refactor the relationship so that my _children_2 array of SECONDGEN_2 objects is held in the appropriate SECONDGEN_1 parent, making the parent relationship "correct"... I've already started doing this. However, I'm intrigued by the error, and wondered if anybody else had encountered it (and how you dealt with it yourself).

Answer 1

This also arises if you compare recursive objects using == instead of ===

If you need to compare actual object instances always use the strict comparison operator === as it only compares if the objects refer to the same instance of the same class.

Short explanation:

If you compare objects using $object == $objectToCompareWith, PHP is comparing every attribute and value of the first object with the second. This comparison is recursive over objects which are properties of the objects being compared.

That means that if both objects share an attribute with an object as its value, PHP does the same == comparison between those attribute objects. Now as soon as on of those attribute objects is recursive (e.g. a self referencing object) the comparison recurses down too until the maximum nesting level is reached.

As stated in the comments by Josh Stuart and mazatwork, strict comparison can be forced when using array functions like in_array() and array_search() by setting their respective $strict parameter to true.

Richard Lord: "Nesting level too deep – recursive dependency?"

PHP Manual: "Comparing Objects"

Answer 2

Looks like a PHP limitation in self-referencing code and trying to display it with print_r, var_dump, var_export, or search through it with in_array. Basically there's no way for those functions to know where to stop recursing if an object is referenced cirularly.

According to this bug report the easiest way to reproduce this is:

$outText = var_export( $GLOBALS, true );
print_r($outText) ;

Other bug reports mention it too, with some more test cases. I'd say that if this is only triggered in var_dump you shouldn't worry too much about it. I definitely second Wrikken's suggestion about xdebug if this is for debugging purposes.

Answer 3

Sometimes (but seldom, as there are limited valid used for such contrustcs) this happens, and as long as your code works properly, I wouldn't give it much thought that a var_dump (a debugging tool, not a production one) cannot cope with it. However, if you still need var_dump to work, I can heartily recommend running xdebug, in which you can set max-depth the var_dump will show, the max-length of a string dump and the maximum amount of children.

Answer 4

I was getting the same error as you but in an entirely different scenario. I'm posting the answer in case anyone else gets here the same way I did.

In the case that you're trying a custom sort (usort) with an array of objects, here's what I had to do:

function cmp($a, $b) {
    if($a->num_estimates == $b->num_estimates) return 0;

    return($a->num_estimates < $b->num_estimates) ? -1 : 1;
}
$c = usort(Company::$companies, "cmp");

It turned out that $object->num_estimates was occasionally returning an object instead of a number. Once I made sure it was always returning a number then the error went away.