Reference problems with PHP classes

Class calculate{
var $var 1=10;
var $var 2=2;
function Add () {
return $this->var1+ $this->var2;
}
function Subtract () {
return $this->var1-$this->var2;
}
function multiplication () {
return $this->var1* $this->var2;
}
}
Class a{
Function ex () {
return Calculate::add ();
}
}
$a = new A;
echo $a->ex ();
?>
Why is the return of 0?

Reply to discussion (solution)

When you statically access a class method, you can only return 0 because the $this is not available
It's good to have no error.

Function ex () {
$t = new Calculate;
return $t->add ();
}
This will return 12.

such as the owner of the moderator or can
Class calculate{
Static $var 1=10;
Static $var 2=2;
function Add () {
Return self:: $var 1+self:: $var 2;
}

}
Class a{
Function ex () {
return Calculate::add ();
}
}
$a = new A;
echo $a->ex ();

Or you can do this:
Define methods and properties as static, which you do not need to instantiate to access.

Class calculate{
static public $var 1 = 10;
static public $var 2 = 2;

public static function add () {
Return self:: $var 1 + self:: $var 2;
}
public static function subtract () {
Return self:: $var 1-self:: $var 2;
}
public static function multiplication () {
Return self:: $var 1 * Self:: $var 2;
}
}
Class a{
Function ex () {
return Calculate::add ();
}
}
$a = new A;
echo $a->ex ();
?>

