I need to allow people to login, signup, etc, and add info to the site, but I can't let them add any odd characters into the form...ie, quotation marks, slashes, semicolons, etc.

How can I get this to work correctly? I've tried htmlspecialchars, but that doesn't achieve what I'm looking for...addslashes and stripslashes don't seem to either - is it easier to simply use JavaScript? If so, can someone help me out with that?