Execution order of PHP

Hi,

In a hobby project I'm relying on the order in which the following piece 
of PHP code is executed:

$client->system->multiCall(
   $client->methodOne(),
   $client->methodTwo()
);

Currently PHP always resolves $client->system (and executes the __get on 
$client) before resolving the arguments to the multiCall() method call.

Is this order something that is specified by PHP and so can be relied 
upon to stay the same in the future or is it just how it currently works.

If it cannot be relied upon to stay this way, I will have to rewrite the 
multiCall method and API...

regards,
Auke van Slooten
Muze