This function does not return a specific value, it's best to simply ignore it.

public OnPlayerCommandText(playerid, cmdtext[])
{
//First the script gets the vehicle id of the current car and then destroys it when typing /destroyveh
if(strcmp(cmdtext, "/destroyveh", true) == 0)
{
new currentveh;
currentveh = GetPlayerVehicleID(playerid);
DestroyVehicle(currentveh);
return 1;
}

return 0;
}