How to call a non static method using static keyword? - php

Question

I am just checking the OOPs static/non-static concept and found something strange.
I have heard that static method's output can be get by using static keyword with resolution operator(::) . But In my program I am getting non static method's value using static keyword. Can any one explain the program? I am getting confused.
<?php
error_reporting(E_ALL);
class parentclass
{
protected function sum()
{
return 145;
}
}
class childclass extends parentclass
{
protected function sum()
{
return 125;
}
}
class grandchild extends childclass
{
function sum()
{
return 100;
}
function __construct()
{
echo static::sum(); // 100 as output but how
}
}
$obj = new grandchild();
?>
Besides this If I am making function sum() of childclass as static like
class childclass extends parentclass
{
protected static function sum()
{
return 125;
}
}
then also it is giving fatal error as following:
Fatal error: Cannot make non static method parentclass::sum() static in class childclass
But why I am not calling that function.

Answer 1

You can call a function statically, even if it is not declared as static, as long as you don't reference $this inside it.
That is why the original code works.
You cannot, however, change the signature of an inherited method.
Fatal error: Cannot make non static method parentclass::sum() static
in class childclass
When you declare protected static function sum() in childclass, you are changing the signature of the inherited method from parentclass which is not specifically declared static.
Bottomline, you are trying to use some PHP quirks that I would recommend against. Yes, they work, but that doesn't mean you should use them.
Stick to a strict style of coding. Write separate methods for static and instance use and call them as intended.

Answer 2

You are using static as a Late Static Binding. But what you heard about was rather
class Foo
{
static function bar() {}
}
$baz = Foo::bar();

Answer 3

I think to understand late static bindings a bit better you could write a variation of your code:
<?php
error_reporting(E_ALL);
class parentclass
{
protected function sum()
{
return 145;
}
public function do_the_math()
{
printf('Static sum: %d, Self sum: %d',
static::sum(),
self::sum());
}
}
class childclass extends parentclass
{
protected function sum()
{
return 125;
}
}
class grandchild extends childclass
{
function sum()
{
return 100;
}
}
$obj = new grandchild();
$obj->do_the_math();
Output:
Static sum: 100, Self sum: 145