references, circular references, oop, and garbage collection in PHP5

[Alan Pinstein <apinstein@xxxxxxx>]

So.. I am having PHP5 memory management problems.

They are similar to those described in this thread:

http://aspn.activestate.com/ASPN/Mail/Message/php-Dev/1555640

(so maybe this question belongs on php-dev but I figured I'd try here  
first... seems like a userland question)

Basically I have an object model to represent db objects, and I am  
bulk-loading the objects via some PHP scripts. Sadly the scripts  
consume unbounded memory because of this problem.

I have done a lot of programming in C++ and Obj-C and the normal way  
to handle circular references is to have parents "retain" (keep ref- 
counted links) to their kids, and have the kids have "weak  
references" (non-ref-counted) links to their parents. This way, when  
the parent is no longer used, it will automatically 0-out the ref  
counts to all children it links too and things GC correctly.

Now, how to do this in PHP?

Well, there are no "documented" weak references. However, I figured  
out by trial that if you obtain a php-reference to an object, it  
doesn't bump the refcount.

Question #1: Is the fact that references to objects in the form  
$objRef = &$obj don't bump the refcount of $obj an intended behavior  
that can be counted on? If so, cool!

So, now that we have a way to do weak references, we should be able  
to implement a reasonable memory management scheme for parent-child  
objects.

Normally from the client side the interface should look something like:

$parent = new Parent();
$child = new Child();
$parent->addChild($child);

Where parent can have 0,n children and child can have 0,1 parent.

And all of parent's internal links to child should be refcounted, and  
the internal links from child to parent are weak (not ref-counted).

So based on the above discovery about references, I tried to  
implement this as such:

class Parent
{
    public $children = array();

    // add a child to our list. We want a ref-counted link here.
    function addChild($child)
    {
        $this->children[] = $child;    // refCounted desired in  
parent->child link
        $child->setParent($this);
    }
}

class Child
{
    public $parent;

    // set the parent object. We want a non-ref-counted link here.
    function setParent(&$parent)
    {
        // refCount NOT desired in child->parent link
        $this->parent = &$parent;
    }
}

Now, you'd expect this would work, but it doesn't. On a hunch, I  
changed the client code to:

$parent = new Parent();
$child = new Child();
$parent->addChild($child);
$child->setParent($child);		// new line here - you can successfully  
create a reference to the object when not passed in as $this

Now, this works! However, it's not practical. The setParent call  
should work from within the parent object....

So what I figured out is that $this is a "pseudo variable" according  
to the docs, but I don't know what that means. Empirically I have  
figured out that it means you cannot create a reference to it.

Is this a feature or a bug? What's the workaround?

This is a serious problem for PHP scripts that need to do things that  
require large amounts of memory.

Please advise.

Thanks,
Alan

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