Static within non-static method is shared between instances - php

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.

Answer 2

I agree that the current PHP documentation is not sufficiently clear on exactly what "scope" means for a static variable inside a non-static method.
It is of course true (as hobodave indicates) that "static" generally means "per class", but static class properties are not exactly the same thing as static variables within a (non static) method, in that the latter are "scoped" by method (every method in a class can have its own static $foo variable, but there can be at most one static class member named $foo).
And I would argue that although the PHP 5 behavior is consistent ("static" always means "one shared instance per class"), it is not the only way that PHP could behave.
For example, most people use static function variables to persist state across function calls, and for global functions the PHP behavior is exactly what most everyone would expect. So it is certainly possible to imagine a PHP interpreter that maintains the state of certain method variables across method invocation and does so "per instance", and that's actually what I also expected to happen the first time I declared a local method variable to be static.

Answer 3

That is what static is, it's the same variable across all instances of the class.
You want to write this so that the variable is a private member of the instance of the class.
class Foo {
private $i = 0;
public function dofoo() {
echo $this->i++ . '<br>';
}
}

Answer 4

The static keyword can be used with variables, or used with class methods and properties. Static variables were introduced in PHP 4 (I think, it might have been earlier). Static class members/methods were introduced in PHP 5.
So, per the manual, a static variable
Another important feature of variable scoping is the static
variable. A static variable exists only in a local function
scope, but it does not lose its value when program execution
leaves this scope.
This is consistant with the behavior you described. If you want a per instance variable, used a regular class member.

Answer 5

Ups 7 years it a long time but anyway here it goes.
All classes have a default constructor why am I saying this?!?
Because if you define a default behaviour in constructor each instance of the class will be affected.
Example:
namespace Statics;
class Foo
{
protected static $_count;
public function Bar()
{
return self::$_count++;
}
public function __construct()
{
self::$_count = 0;
}
}
Resulting in:
require 'Foo.php';
use Statics\Foo;
$bar = new Foo();
echo $bar->bar().'<br>';
echo $bar->bar().'<br>';
echo $bar->bar().'<br>';
$barcode = new Foo();
echo $barcode->bar().'<br>';
echo $barcode->bar().'<br>';
echo $barcode->bar().'<br>';
0
1
2
0
1
2
Every new instance from the upper class will start from 0!
The static count behaviour will NOT be shared across the multiple instances as it will be starting from the value assigned in constructor.
If you need to share data across multiple instances all you need to do is to define a static variable and assign default data outside the constructor!
Example:
namespace Statics;
class Foo
{
//default value
protected static $_count = 0;
public function Bar()
{
return self::$_count++;
}
public function __construct()
{
//do something else
}
}
Resulting in:
require 'Foo.php';
use Statics\Foo;
$bar = new Foo();
echo $bar->bar().'<br>';
echo $bar->bar().'<br>';
echo $bar->bar().'<br>';
$barcode = new Foo();
echo $barcode->bar().'<br>';
echo $barcode->bar().'<br>';
echo $barcode->bar().'<br>';
0
1
2
3
4
5
As you can see the results are completely different, the memory space allocation is the same in between class instances but it can produce different results based on how you define default value.
I hope it helped, not that the above answers are wrong but I felt that it was important to understand the all concept from this angle.
Regards, from Portugal!