[renatog]

PHP Code:

public OnPlayerEnterVehicle(playerid, 598, 0) 


You must provide the parameter names:

PHP Code:

OnPlayerEnterVehicle(playerid, vehicleid, ispassenger) 


Then you use the values in your code inside the callback's body:

PHP Code:

OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{ 
    if(GetVehicleModel(vehicleid) == 598 && !ispassenger)
    {
        SetPlayerWantedLevel(playerid, 6); 
        SendClientMessage(playerid, RED, "Law enforcement vehicle theft [WANTED LEVEL INCREASED]"); 
        return 1; 
    }
}