PHP Filtering Form Input

Hello I have a simple form on my site that allows users to send feedback. The form is validated开发者_开发百科 in more than one way sometimes I get emails that are just a bunch of gibberish like: sadfasdasd sadfsa dasd werqwer gsdfh hdfgh.

Question: Is there any clever way to check input string to prevent this?

Distinguishing between rubbish and sense is a very hard job for a computer and apparently not easy for humans either. The only way to do that with any degree of success is to use some sort of spam filtering software.