`static` keyword inside function?

Question

I was looking at the source for Drupal 7, and I found some things I hadn't seen before. I did some initial looking in the php manual, but it didn't explain these examples.

What does the keyword static do to a variable inside a function?

function module_load_all($bootstrap = FALSE) {
    static $has_run = FALSE

Answer 1

static variable in a function means that no matter how many times you call the function, there's only 1 variable.

<?php

class Foo{
    protected static $test = 'Foo';
    function yourstatic(){
        static $test = 0;
        $test++;
        echo $test . "\n"; 
    }

    function bar(){
        $test = 0;
        $test++;
        echo $test . "\n";
    }
}

$f = new Foo();
$f->yourstatic(); // 1
$f->yourstatic(); // 2
$f->yourstatic(); // 3
$f->bar(); // 1
$f->bar(); // 1
$f->bar(); // 1

?>

Answer 2

Seems like nobody mentioned so far, that static variables inside different instances of the same class remain their state. So be careful when writing OOP code.

Consider this:

class Foo
{
    public function call()
    {
        static $test = 0;

        $test++;
        echo $test . PHP_EOL; 
    }
}

$a = new Foo();
$a->call(); // 1
$a->call(); // 2
$a->call(); // 3


$b = new Foo();
$b->call(); // 4
$b->call(); // 5

If you want a static variable to remember its state only for current class instance, you'd better stick to a class property, like this:

class Bar
{
    private $test = 0;

    public function call()
    {
        $this->test++;
        echo $this->test . PHP_EOL; 
    }
}


$a = new Bar();
$a->call(); // 1
$a->call(); // 2
$a->call(); // 3


$b = new Bar();
$b->call(); // 1
$b->call(); // 2

Answer 3

Static works the same way as it does in a class. The variable is shared across all instances of a function. In your particular example, once the function is run, $has_run is set to TRUE. All future runs of the function will have $has_run = TRUE. This is particularly useful in recursive functions (as an alternative to passing the count).

A static variable exists only in a local function scope, but it does not lose its value when program execution leaves this scope.

See http://php.net/manual/en/language.variables.scope.php

Answer 4

It makes the function remember the value of the given variable ($has_run in your example) between multiple calls.

You could use this for different purposes, for example:

function doStuff() {
  static $cache = null;

  if ($cache === null) {
     $cache = '%heavy database stuff or something%';
  }

  // code using $cache
}

In this example, the if would only be executed once. Even if multiple calls to doStuff would occur.

Answer 5

Given the following example:

function a($s){
    static $v = 10;
    echo $v;
    $v = $s;
}

First call of

a(20);

will output 10, then $v to be 20. The variable $v is not garbage collected after the function ends, as it is a static (non-dynamic) variable. The variable will stay within its scope until the script totally ends.

Therefore, the following call of

a(15);

will then output 20, and then set $v to be 15.

Answer 6

To expand on the answer of Yang

If you extend a class with static variables, the individual extended classes will hold their "own" referenced static that's shared between instances.

<?php
class base {
     function calc() {
        static $foo = 0;
        $foo++;
        return $foo;
     }
}

class one extends base {
    function e() {
        echo "one:".$this->calc().PHP_EOL;
    }
}
class two extends base {
    function p() {
        echo "two:".$this->calc().PHP_EOL;
    }
}
$x = new one();
$y = new two();
$x_repeat = new one();

$x->e();
$y->p();
$x->e();
$x_repeat->e();
$x->e();
$x_repeat->e();
$y->p();

outputs:

one:1
two:1
one:2
one:3 <-- x_repeat
one:4
one:5 <-- x_repeat
two:2

http://ideone.com/W4W5Qv