Calling non-static method with double-colon(::)

Solution 1:

PHP is very loose with static vs. non-static methods. One thing I don't see noted here is that if you call a non-static method, ns statically from within a non-static method of class C, $this inside ns will refer to your instance of C.

class A 
{
    public function test()
    {
        echo $this->name;
    }
}

class C 
{
     public function q()
     {
         $this->name = 'hello';
         A::test();
     }
}

$c = new C;
$c->q();// prints hello

This is actually an error of some kind if you have strict error reporting on, but not otherwise.

Solution 2:

This is a known "quirk" of PHP. It's by design to prevent back-propagation for figuring out if some time ago we actually instantiated an object or not (remember, PHP is interpreted, not compiled). However, accessing any non-static member the via scope resolution operator if the object is not instantiated will issue a fatal error.

Courtesy of PHP.net:

class User {
    const GIVEN = 1;  // class constants can't be labeled static nor assigned visibility
    public $a=2;
    public static $b=3;

    public function me(){
        echo "print me";
    }
     public static function you() {
        echo "print you";
    }
}

class myUser extends User {
}

// Are properties and methods instantiated to an object of a class, & are they accessible?
//$object1= new User();        // uncomment this line with each of the following lines individually
//echo $object1->GIVEN . "</br>";        // yields nothing
//echo $object1->GIVE . "</br>";        //  deliberately misnamed, still yields nothing
//echo $object1->User::GIVEN . "</br>";    // yields nothing
//echo $object1->a . "</br>";        // yields 2
//echo $object1->b . "</br>";        // yields nothing
//echo $object1->me() . "</br>";        // yields print me
//echo $object1->you() . "</br>";        // yields print you

// Are  properties and methods instantiated to an object of a child class,  & are accessible?
//$object2= new myUser();        // uncomment this line with each of the following lines individually
//echo $object2->GIVEN . "</br>";        // yields nothing
//echo $object2->a . "</br>";        // yields 2
//echo $object2->b . "</br>";        // yields nothing
//echo $object2->me() . "</br>";        // yields print me
//echo $object2->you() . "</br>";        // yields print you

// Are the properties and methods accessible directly in the class?
//echo User::GIVEN . "</br>";        // yields 1
//echo User::$a . "</br>";            // yields fatal error since it is not static
//echo User::$b . "</br>";            // yields 3
//echo User::me() . "</br>";        // yields print me
//echo User::you() . "</br>";        // yields print you

// Are the properties and methods copied to the child class and are they accessible?
//echo myUser::GIVEN . "</br>";        // yields 1
//echo myUser::$a . "</br>";        // yields fatal error since it is not static
//echo myUser::$b . "</br>";        // yields 3
//echo myUser::me() . "</br>";        // yields print me
//echo myUser::you() . "</br>";        // yields print you
?>

Solution 3:

This is PHP 4 backwards compatibility. In PHP 4 you could not differ between an object method and the global function written as a static class method. Therefore both did work.

However with the changes in the object model with PHP 5 - http://php.net/oop5 - the static keyword has been introduced.

And then since PHP 5.1.3 you get proper strict standard warnings about those like:

Strict Standards: Non-static method Foo::bar() should not be called statically

And/Or:

Strict Standards: Non-static method Foo::bar() should not be called statically, assuming $this from incompatible context

which you should have enabled for your development setup. So it's merely backwards compatibility to a time where the language couldn't differ enough so this was "defined" at run-time.

Nowadays you can define it already in the code, however the code will not break if you still call it "wrong".

Some Demo to trigger the error messages and to show the changed behavior over different PHP versions: http://3v4l.org/8WRQH

Solution 4:

PHP 4 did not have a static keyword (in function declaration context) but still allowed methods to be called statically with ::. This continued in PHP 5 for backwards compatibility purposes.

Solution 5:

Warning In PHP 7, calling non-static methods statically is deprecated, and will generate an E_DEPRECATED warning. Support for calling non-static methods statically may be removed in the future.

Link