Static within non-static method is shared between instances

Question

I've come across some unexpected behavior with static variables defined inside object methods being shared across instances. This is probably known behavior, but as I browse the PHP documentation I can't find instances of statically-defined variables within object methods.

Here is a reduction of the behavior I've come across:

<?php

class Foo {
  public function dofoo() {
    static $i = 0;
    echo $i++ . '<br>';
  }
}

$f = new Foo;
$g = new Foo;

$f->dofoo(); // expected 0, got 0
$f->dofoo(); // expected 1, got 1
$f->dofoo(); // expected 2, got 2

$g->dofoo(); // expected 0, got 3
$g->dofoo(); // expected 1, got 4
$g->dofoo(); // expected 2, got 5

Now, I would have expected $i to be static per instance, but in reality $i is shared between the instances. For my own edification, could someone elaborate on why this is the case, and where it's documented on php.net?

Answer 1

This is the very definition of static.

If you want members to be specific to an instance of an object, then you use class properties

e.g.

<?php

class Foo
{
    protected $_count = 0;
    public function doFoo()
    {
        echo $this->_count++, '<br>';
    }
}

Edit: Ok, I linked the documentation to the OOP static properties. The concept is the same though. If you read the variable scope docs you'll see:

Note: Static declarations are resolved in compile-time.

Thus when your script is compiled (before it executes) the static is "setup" (not sure what term to use). No matter how many objects you instantiate, when that function is "built" the static variable references the same copy as everyone else.