"If player has" Scripting

[Fysez]

I'm trying to make it so that "If this player has this weapon, Then you can do this".
For instance, You find a guy. This guy gives you a key. You go up to a door (without this key) and it doesn't work.
But you go up to it (with this key) and it opens linking to another level.

That was just an example. I want this to work for many things like, You buy this item, And you are then able to have access to a room.

All I can think of script-wise is
findweapon("key");

then I have NO idea at ALL what script would come come after for searching this players weapons. It's got to be like, Find the weapon, If found, The door will open, If not, it will say "You are not Authorized" or something.
Help? I have no clue what to do..