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

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);
    }
}