Your code is useless and wrong, Sn!ff3r. Wrong in the way of you should do a check to see if max_clients is already saved ( better to save it out of this function though ) and is_user_connected is not needed. And using a macro would be more appropriate.
Code:
new max_clients ;
[...]
max_clients = get_maxplayers()
[...]
#define IsPlayer(%1) ( 1 <= %1 <= max_clients )