lwgames: Your php code is requiring to md5 hash 3 concatenated elements.
(The "." is the equivalent of "&" in Construct2 as far as string manipulation goes.)
In your URL ends that would be "" & MD5(var1 & var2 & key)
Where "var1", "var2" and "key" are Construct2 variables.
If this doesn't work, try "" & MD5B(var1 & var2 & key) which is about the same but the output is BASE64 so a "secured"/encrypted URL.
Here your issue is not with the algorithm but with the PHP I'd say.