Static variables inside member functions of a classes have wrong scope. They should belong to each instance of a class, like normal class properties, but they're belong to class definition instead and, therefore, shared between every instance of this class. It makes static variables unusable into class member variables. 
 Here is an example:

// Define class
class A
{
    var $id = null;

    function A($id)
    {
        $this->id = $id;
    }

    function func()
    {
// Here we have counter into static variable
        static $cnt;
        if (!$cnt)
            $cnt = 0;
// Show counter value and ID of a class instance
        echo $this->id.": ".($cnt++)."\n";
    }
};

// Create 2 instances of our class with different IDs
$a = new A('A');
$b = new A('B');
// Call method of each class instance twice and see, 
// what will happen
$a->func();
$b->func();
$a->func();
$b->func();

Expected results:
-----------------
A: 0
B: 0
A: 1
B: 1

Actual results:
---------------
A: 0
B: 1
A: 2
B: 3