The md5 function and algorithm is a one-way encryption process. If you want to decode an md5 string (which I don't know where you got from or you're legally fit to use it) you would have to create a so called "rainbow-table" (http://en.wikipedia.org/wiki/Rainbow_table), which can take more than a person's lifetime to generate all possible inputs.
The other posters are correct - you cannot decrypt an md5. However, if you know what the correct value is and are trying to match it in a database, convert your string to md5 (using 'md5($string);') and then see if that md5 matches the correct one in your database. Now, as previously stated, if you don't know the correct value, and try this approach for all possible alphanumeric values (plus those with symbols), it would be called a "brute force" approach.
there is no direct function to convert md5 but there are many website available which has library of md5 passwords. search on google for those website and check is your password (md5 encrypted string) is there or not.
There are some online databases with millions of md5 hashes with their respective value. Just search for 'md5 decrypt'. by the way. Here is a little tool to encrypt text to md5 http://wihee.com/text2md5 ~Mike
You can't Decode MD5 what you can do to Check is check any online Database OR make md5 of text and check if its Same then you will know the Pass is same.
A number of rainbow tables are indexed by google -- sometimes you can just type the MD5'd code into google search, and one of the results gives you the value. Every two or three years someone using that as a legitimate attack avenue crops up in the news.