PHP: Why can't a static variable in a class be used as a variable function?

Question

I'm new to using static methods & properties in classes. What I'm trying to do is run a variable function, but can't use:

self::$static_var()

PHP throws a notice:

Undefined variable: static_var

I have to first assign to a local variable like so:

$local_var = self::$static_var;

Then I can do

$local_var();

Here's some example code. I don't understand why Test 1 doesn't work. I have to do Test 2 in order to get the desired functionality. Question: Why is it that Test 1 doesn't work?

Test 1 - doesn't work

X::do_stuff('whatever');

class X {
    public static $static_var = 'print_r';

    public static function do_stuff($passed_var) {
        self::$static_var($passed_var);
    }
}

Test 2 - works

X::do_stuff('whatever');

class X {
    public static $static_var = 'print_r';

    public static function do_stuff($passed_var) {
        $local_var = self::$static_var;
        $local_var($passed_var);
    }
}

Answer 1

Use call-user-func:

call_user_func(self::$static_var, $passed_var);

Concerning your edited question:

I tried to find an explanation in PHP docs. It is probably because $static_var is not yet evaluated when the function call is processed. But the best answer to your question is probably: because it's the way it is. A good example is: $classname::metdhod(); was not valid before PHP 5.3. Now it is. There is really no reason why. You should ask the PHP guys.