test1.php contains :
<?php
class a {
  function MyFunct($i, $j, $k='defaut_k') {
    echo " in a ";
  }
}
?>

test2.php contains
<?php
include("test1.php");

class b extends a {
  function MyFunct($i, $j, $k='defaut_k') {
    echo " in b ";
    parent::MyFunct($i,$j,$k); // does not work
// got : Fatal error: No parent class available in this context in ...
    a::MyFunct($i,$j,$k); // works
  }
}

whereas first example presented here works fine for me :
http://www.php.net/manual/en/keyword.parent.php