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To be frank - you do not want to decrypt a password. Even if you had a working solution, it would greatly harm your security.
Instead, follow Robber's suggestion.
The crypt function would work really well. The following solution uses the first two letters of the password as salt. Finally, it only returns the last 11 characters (since the first two are the salt as plain text:
$cryptpwd = substr(crypt($pwd,substr($pwd,0,2)),2);
Now, store this value in your database. When someone logs in, simply encrypt the password the same way and compare it to the database value.