[PHP] How can I check for characters in a $_POST[] variable?

[Eric]

I have this problem when using php because my computer recognizes
the characters "." and ".." as an existing file when I use file_exists. Also
I want to check $_POST["username"] for characters other then A-Z a-z and 0-9.
If it contains anything other then, I would like to prompt the user but
I can't seam to use foreach properly and I don't know how to itterate
through the post variable with a for loop while loop or do while loop.