Behavior of static property in PHP

Question

I'm trying to understand how static property works.My example:

class Model_Cart{
    static public $_a;
}

I created object of this class in different scripts. Script 1:

 Model_Cart::$_a = true;
 var_dump(Model_Cart::$_a);

Output is "true".

But if I execute second script after:

var_dump(Model_Cart::$_a)

Output is NULL.

I expected that static variable is shared beetwen all instance of class. Can you explain this behaviour?

Answer 1

You cannot share variables across requests - they would need to be either send via a POST or GET request.

Answer 2

The behaviour is as expected actually. Please take note that you cannot - without the use of sessions, cookies or a database of some sort - share properties or values between requests. That's because http is a so called 'stateless protocol', which pretty much means that with every request the application is build up again from the ground up.

Please take a note at the following examples:

script_number_one.php

$my_var = 'hello world!';
echo $my_var; // does what you think it does

script_number_two.php

echo $my_var; // echoes nothing, or raises a warning that the var doesn't exist

As you can see it doesn't matter what you do in script one, as script two just doesn't know about no 1 (and doesn't care either actually).

Pretty much the same happens with your class. But you can do something else though, and this is probably what you did expect:

class myStaticClass {
    public static $static_var = 'Hello world!';

    public function alter_static_value($new_value) {
        self::$static_var = $new_value;
    }

}

$_obj_1 = new myStaticClass;
$_obj_2 = new myStaticClass;

echo $_obj_1::$static_var; // echoes 'Hello World!'
echo $_obj_2::$static_var; // also echoes 'Hello world!'

$_obj_1->alter_static_value('Bye world!');
echo $_obj_2::$static_var; // echoes 'Bye world!';

As you can see, a static value isn't particular for an object instance, but is specific for the whole class, and thus accessible for all objects that instantiated that particular class.

Hope this helps.