Ever seen on Minecraft for Xbox and PC people can build mob spawners so zombies etc just come right out of it? Well, you can build a proper mob spawner of Pocket Edition, but you can make a fairly good make-shift one. These instructions will also show you how to make a trap for mobs, too.
Step 1: Build a fairly large room completely out of cobblestone.
Step 2: Make a 2x2 hole on one side of the building (not the roof!).
Step 3: Above the hole, place 2x2 blocks (if this step confuses you, here is a picture to show you what I mean).
Step 4: Under the thing-you-made-in-step-three, make a hole three blocks deep. Make it the same width/length of the thing-you-made-in-step-three. Still confused? Here is a picture to show you.
Step 5: Put a fence around the hole to stop spiders from escaping.
Step 6: That's pretty much it! Just to be sure, here is an image of a mob spawner I made. If it doesn't look similar to this, there is something not right about it.
Also, here is some images of zombies and stuff trapped inside the hole.
Enjoy your mob spawner!
(If anything on the game changes to improve/change the mob spawner, I will update this post about it.)