Question

as far as i know, :: is using for calling static functions and base class functions in a subclass. and as far as i know, usually we have to create an instance of a class for using it out of the class.

class a 
{
    public function foo()
    {
       //
    }
}

for using this class:

$instance = new a();
$instance->foo();

but its possible that we call the foo function without creating any instance and only using ::. for example the following code is written out of class and works well:

a::foo();

why does it work? and how?

Answer 1

:: is the scope resolution operator.

http://php.net/manual/en/language.oop5.paamayim-nekudotayim.php

From PHP’s docs:

<?php
class MyClass {
    const CONST_VALUE = 'A constant value';
}

$classname = 'MyClass';
echo $classname::CONST_VALUE; // As of PHP 5.3.0

echo MyClass::CONST_VALUE;
?>

It is like ->, but has some special semantics.