Fetching method from other file

I have a issue with PHP that I just cant figure out. I have a method in a file called dbmethods.php, that I am trying to invoke from another file (lets call it index.php). This is the method.

function getAllDevicesFromUser($userID){
  $query = $db->prepare("SELECT * FROM devices WHERE UserID = " . $userID);
  $query->execute();
  $devices = $query->fetchAll();
  return $devices;
}

So the first thing you might question is the $db variable. This is again included from another file called database.php. Now the weird thing is that this $db variable seems to work in other cases (I use it frequently), but not in this case. I have tried putting a echo statement before the first statement in the method and it echoes it out, but not if I try after the first statement.

So it is this line that creates the problem in my opinion.

 $query = $db->prepare("SELECT * FROM devices WHERE UserID = " . $userID);

Now, the string could cause the problem, but I have tested it and it is a valid string. So I think it has to do with the $db variable, but I have no idea what that might be.

Anyone want to has any idea?