How to call non-static method from static method of same class?

You must create a new object inside the static method to access non-static methods inside that class:

class Foo {

    public function fun1()
    {
        return 'non-static';
    }

    public static function fun2()
    {
        return (new self)->fun1();
    }
}

echo Foo::fun2();

The result would be non-static

Later edit: As seen an interest in passing variables to the constructor I will post an updated version of the class:

class Foo {

    private $foo;
    private $bar;

    public function __construct($foo, $bar)
    {
        $this->foo = $foo;
        $this->bar = $bar;
    }

    public function fun1()
    {
        return $this->foo . ' - ' . $this->bar;
    }

    public static function fun2($foo, $bar)
    {
        return (new self($foo, $bar))->fun1();
    }
}

echo Foo::fun2('foo', 'bar');

The result would be foo - bar

The main difference would be that you can call static methods for a class without having to instantiate an object of that class. So, in your static method try

Foo $objInst = new Foo();
$objInst->fun1();

But I don't see how this would make any sense in any context.