PHP can call non-static methods through the class name

Today, there is a brother encounter a problem, that is, you can call the class directly from the function, I test, it is true, the concept is only a static method can be called, and now be refreshed, so I added a line in the method $this related operations, and then run, immediately error, That is, PHP does not have strict restrictions when invoking a method, but when executed inside the method, it is thrown when it encounters the variable $this: Using $this When not in object context
That is, if your method does not involve the instantiation of this kind of reference, you can directly call the

Here is the test code:
T::t1 ();

Class t{
Private $data;

public static function T1 () {
echo "\ n";
Echo __method__;
echo "\ n";
SELF::F1 ();
}

public static function T2 () {
echo "\ n";
Echo __method__;
echo "\ n";
}

Public Function F1 () {
$this->data = ' Jken ';
echo "\ n";
Echo __method__;
echo "\ n";
}
}
?>