You can create users on Firebird/Interbase this way:
<?php
    if (($service = ibase_service_attach('localhost', 'sysdba', 'masterkey')) != FALSE) {
        $server_info  = ibase_server_info($service, IBASE_SVC_SERVER_VERSION) 
                      . ' / '
                      . ibase_server_info($service, IBASE_SVC_IMPLEMENTATION);
        }
    else {
        $ib_error = ibase_errmsg();
    }
echo $server_info;
if (($result = ibase_add_user($service, 'pippo', 'pp')) != FALSE) {
        echo 'User created';
        ibase_service_detach($service);
    }
    else {
        $ib_error = ibase_errmsg();
        ibase_service_detach($service);
    }  
?>