[PHP] decrypting passwords

[Steph]

User enters a new password through a form.  Then it is put in the table by the 
following.  ($userid is the username)

$query = mysql_query("update users set password = encrypt('$password','$userid') where 
userid = '$userid'")
 or die("Couldn't execute query.");

I need to know how to decrypt the password so that I can have it emailed to the user 
should they forget it.

Any help would be appreciated

Thanks Steph