In PHP >= 4.3.0, if you use additional reference parameters, you will get this warning:
PHP Warning: Call-time pass-by-reference has been deprecated - argument passed by value
This clearly has the potential to cause unexpected consequences (vars left empty), and will break existing code. So don't do it! These docs need updating to say this too.
The syntax:
list($a, $b) = sscanf("hello world", "%s %s");
will work as expected, and doesn't seem to cause any problems with Apache that I've noticed.