The callback
syntax is a little odd in PHP. What you need to do is make an array. The 1st element is the object, and the 2nd is the method.
call_user_func(array($player, 'SayHi'));
You can also do it without call_user_func
:
$player->{'SayHi'}();
Or:
$method = 'SayHi';
$player->$method();