function changed in php 5! in earlier versions the syntax was:
bool ibase_modify_user ( string server, string dba_user_name, string dba_user_password, string user_name, string password [, string first_name [, string middle_name [, string last_name]]])
now it's necessary to connect to service manager, then call ibase_modify_user() and then disconnect from service manager:
$ibserv = ibase_service_attach($host, $dba_username, $dba_password);
ibase_modify_user($ibserv, $user_name, $password);
ibase_service_dettach($ibserv);