On the contrary to what everyone is saying...believe me when I say that I have reliable sources, and mirrors can be used for many things. Scrying, portals, and much more. But specifically about portals...
Mirrors are doors to another world, yes. But...be careful as they are not always the most trustable things. Often times they are portals for other entities, including demons, spirits, and polterguistes. And they can easily fool you. You must have very talented skills at scrying and using aura to look into the future or "open a portal". Physically, I do not for sure know if it is possible to go through mirrors using them as portals or not. However, I highly suspect that it's more of a mental thing than physical. May I ask what you are trying to open a portal for?