php - Calling static class outside of namespace

Inside a class "cl1" in the namespace "space1", there is the function "fctn1", and inside it I want to call the static method "fctn2" from the class "cl2". The class cl2 is not inside any namespaces.

<?php
namespace space1;

class cl1
{
    public function fctn1()
    {
        cl2::fctn2();
    }
}

<?php

class cl2
{
    public static function fctn2()
    {
        // stuff here
    }
}

How can I call fctn2 inside the fctn1 on PHP?

I tryied simply calling the method inside the class just like shown in the code on the question.

Solution:

Honk der Hase gave a satisfactory answer in a comment:

Prepend a backslash to address the global namespace:

\cl2::fetch2();