Here statically accessed property prefer property of the class for which it is called. Where as self keyword enforces use of current class only. Refer the below example:

<?php
class a{

static protected $test="class a";

public function static_test(){

echo static::$test; // Results class b
echo self::$test; // Results class a

}

}

class b extends a{

static protected $test="class b";

}

$obj = new b();
$obj->static_test();
?>