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PHP 5.2 - Nesting level too deep - recursive dependency?

13th November 2006

I installed PHP 5.2 on one of my testing servers today and a couple of bits of code that previously worked fine in version 5.1.6 threw fatal errors in the new version. The error message was “Nesting level too deep - recursive dependency?” and it took a little time to track down the root of the problem. Here’s what I’d done wrong.

In PHP there are two comparison operators, == and ===. It’s generally known that the first is not strict about type but the second is. So, for example

echo ( false == 0 ); // true
echo ( false === 0 ); // false
    - 0 is an integer and false is a boolean

My problem arose from using non-strict typing with objects.

$a = new MyObj();
$b = new MyObj();
if( $a == $b )
...

I hadn’t considered what I was doing with this code. When comparing two objects using the non-strict comparison operator (==) PHP compares all the properties of the objects and if they match the objects are deemed to be equal. If they don’t match they are not equal. In effect, we have a recursive comparison of all the properties of each object, and all their properties, etc. until we reach basic data types like strings and integers.

If, however, we use strict comparison (===), PHP will check whether the two objects are exactly the same object, not just objects with the same properties.

class MyObj
{
    public $p;
}

$a = new MyObj();
$b = new MyObj();
$c = new MyObj();
$a->p = 1;
$b->p = 1;
$c->p = 2;
echo ( $a == $c ); // false
echo ( $a == $b ); // true
echo ( $a === $b ); // false

The problem arises if you have circular references in your objects properties. So, for example

class MyObj
{
    public $p;
}
class OtherObj
{
    public $q;
}

$a = new MyObj();
$b = new OtherObj();
$a->p = $b;
$b->q = $a; // the circular reference: $a->p->q === $a

$c = new MyObj();
$d = new OtherObj();
$c->p = $d;
$d->q = $c;// another circular reference: $c->p->q === $c

echo ( $a == $c ); // Fatal error:
    Nesting level too deep - recursive dependency?

In order to compare $a to $c, PHP must compare their properties. So the logic in PHP goes something like this: $a == $c if $a->p == $c->p if $a->p->q == $c->p->q if $a->p->q->p == $c->p->q->p etc. indefinitely.

PHP 5.1 seemed to smooth over the problem somehow (probably after a certain level of recursion it simply returned false) - and usually it worked out fine. PHP 5.2 correctly produces the fatal error above.

Once you know the problem, the solution is easy - use strict comparison.

echo ( $a === $c ); // false (and no error)

The strict comparison will simply check whether the two objects are at the same location in memory and so doesn’t even look at the values of the properties.

N.B. The same problem can arise when using the negated comparison operators (use !== instead of !=) and when using in_array (use in_array’s third parameter to indicate strict comparison).

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15 Comments

  • you saved my day. thanks man

    greenone  |  8th December 2006 at 5:31 am

  • Indeed, very helpfull tip, thanks

    Willem  |  15th December 2006 at 1:51 pm

  • Thanks!!

    nuxodin  |  31st December 2006 at 7:49 pm

  • Thanks for the tip. I went over the change log and the update info before upgrading to php 5.2 hoping to find breaking changes before upgrading.
    This issue wasn’t mentioned anywhere.

    Shahar  |  1st January 2007 at 11:20 am

  • I have supplied Ilia with a distilled overview of your analysis to be added to the Upgrading guide with the next release. Thanks for this, and in the future do not hesitate to contact internals if you find stuff like this not being mentioned in the Upgrading guide.

    Lukas  |  1st January 2007 at 11:50 pm

  • I upgraded my server late last night and didn’t realise it would cause any problems until clients rang the next morning saying their site was broken.

    Thanks for posting your information, it saved me hours of troubleshooting!

    Kenny  |  1st February 2007 at 1:47 am

  • Just another thanks I was banging my head against a wall for an hour with problems caused by this stupid issue…

    Keith  |  6th March 2007 at 7:20 am

  • Thanks a lot :-) that bug had me stumped - and all I needed was a single extra character!

    Nad  |  14th March 2007 at 2:10 am

  • Indeed, +1 person who’s day you’ve saved :)

    tolan  |  21st March 2007 at 4:31 pm

  • Thank you for the tip, saved me hours of possible debugging.

    loconet  |  6th April 2007 at 8:13 pm

  • Big thanks for this. I hit the same problem with array_search and adding the strict parameter fixed it.

    I didn’t really know what strict did before. Now that I do I’ll use it always - it’s got to be faster than comparing all the properties.

    Rick Mason  |  9th May 2007 at 8:41 pm

  • Very useful, thanks. (I was wondering how such a bug could have stayed unknown, I’m glad to know it’s simply because of upgrading PHP.)

    a3_nm  |  19th June 2007 at 10:49 am

  • Indeed,Thank you for the tip

    michael  |  29th June 2007 at 5:32 pm

  • this error also occurs with the Get_Browser() function as well as some PREG functions.

    Chris  |  9th July 2007 at 2:19 pm

  • Thanks a lot! Like you, I just upgraded my PHP and began to get this error. Thanks to your post, this issue has been resolved in my programs. Thanks again!!

    Tom  |  27th July 2007 at 8:13 pm

Comments closed.