Question

What are the use of static functions in OOP?

How it is differentiated from other function ?

Answer 1

As a very simple, somewhat contrived example:

class Foo {

    protected $bar = null;
    protected $baz = null;

    public function __construct($bar, $baz) {
        $this->bar = $bar;
        $this->baz = $baz;
    }

    public static function instantiateFromArray(array $data) {
        return new self($data['bar'], $data['baz']);
    }

}

$foo = new Foo('bar', 'baz');
// or
$foo = Foo::instantiateFromArray(array('bar' => 42, 'baz' => 'nine'));

In this case the static function serves as an alternative constructor, allowing you to construct the object from an array of data instead of separate arguments.

Generally, static functions provide functionality around an object without needing to instantiate it. There are many uses for that. At the very least, entirely static classes which are never instantiated can be used to bundle functions and related data together, which in itself makes code cleaner. That’s where they differ from normal functions: normal functions cannot save “external” data (without using globals, which you don’t want to do), static class methods can save data in static class properties.