The params[] array is a global variable which always contains the parameters passed to a script call, either from outside (like onActionServerside) or by calling the function of another object. The params[] variable is not containing the parameters passed to the function. That's why you need to save it to a temporary variable if you want to use it later.
If the onActionServerSide function always expects the same number of parameters, you can also name the parameters in the function-declaration, then you don't need to copy them to temporary variables yourself. Something like this:
PHP Code:
function onActionServerside(chat) {
echo("player chat: " @ chat);
}