PHP Fatal error Using this when not in object context

Question

I am working on a new WebApp without a Framework. 

In my index.php I'm using: require_once('load.php');

And in load.php I'm using require_once('class.php'); to load my class.php.

I am getting this error in my class.php:

Fatal error: Using $this when not in object context in class.php on line ... (in this example it would be 11)

class.php :

class foobar {

    public $foo;

    public function __construct() {
        global $foo;

        $this->foo = $foo;
    }

    public function foobarfunc() {
        return $this->foo();
    }

    public function foo() {
        return $this->foo;
    }
}

In my index.php I'm loading maybe foobarfunc() like this:

foobar::foobarfunc();

Can someone please help me with this error?