It’s not impossible to hang a mirror on a hollow door but it can be a bit challenging; however, do not worry, there are special bolts designed for hanging objects on hollow doors such as—
Hollow Door and Drywall Anchors manufactured by E-Z Ancor
Hollow-Door Anchor Toggle TA bearing the brand name "TOGGLER".
That's just to name a couple of them that you can purchase at your local hardware, your home improvement store, or online.
Step 1 – Decide where to Position Your Mirror on the Hollow Door
Hold your mirror up to the door to determine the best position. Make sure it’s the right height. If the mirror is heavy, you may need to use two hooks.
Step 2 – Get the Right Measurements of the Positions of the Holes
Draw light lines on the door using a pencil to help you get the correct measurements of the placements where holes will be drilled. If you plan to use two hooks, make sure the mirror is level before marking the spots with your pencil.
A laser level can be used to make sure your marks are lined up correctly. The level can be placed on the wall via its attached suction cup; adjust until the level’s bubble is in the middle. Turn on the laser light to project a line across the wall and double-check your pencil markings.
Step 3 – Drill Holes on the Hollow Door
Drill holes on where you marked it on the door. Make sure that the holes are big enough for the toggle bolt to fit. Drilling is necessary so the door will not crack but make sure not to drill through the entire door.
Step 4 – Put the Toggle Bolts through the Holes
Using a screwdriver, place and tighten the toggle bolt in the holes. The toggle bolt’s back should grasp the hollow door’s insides once the screw is tightened.
Step 5 – Hang the Mirror
The last step of course is to hang the mirror on the hollow door.