gistfile1.php

<?php

class C1 {
    function func_1()
    {
        print 'Function func_1() call';
    }
}

class C2 {
    function func_x()
    {
        C1::func_1();
    }
}

class C3 {
    static function func_y()
    {
        C1::func_1();
    }
}

$x = new C2();
$x->func_x();
C3::func_y();