Al foil may burn the plant. Any white gloss material will work better.
That's a bunch of internet bulls@#t.
Al foil has a few disadvantages (its fragile and conducts electricity), but this idea that light is going to be magically magnified bouncing off of it and burn plants is absurd.
Mylar. . .which everyone touts as the best reflective stuff. . .is just aluminized plastic sheeting.
If you want to use white reflective stuff, go ahead. If you want to use mylar, go ahead. But if all you're got is AL foil, that's still OK.
The inside of your PC case is probably aluminum.
So if you REALLY want to be dastardly clever, get a can of Mother's aluminum polish, and a buffing wheel, and you can just polish the inside of your PC to a mirror-like finish!