Simply, static functions function independently of the class where they belong.
$this means, this is an object of this class. It does not apply to static functions.
class test {
public function sayHi($hi = "Hi") {
$this->hi = $hi;
return $this->hi;
}
}
class test1 {
public static function sayHi($hi) {
$hi = "Hi";
return $hi;
}
}
// Test
$mytest = new test();
print $mytest->sayHi('hello'); // returns 'hello'
print test1::sayHi('hello'); // returns 'Hi'