Check the Event Log.

Parameter name should be 0, not @0, in the parameter mapping screen. All of that declare and set stuff should not be required. It really should be as simple as 

I do not know PHP, so cannot be of much help.
Are you able to detect exactly which line of code generates the error?
Is there a reason why exactly...

This sort of thing:

I've just checked a few of my packages and noticed that I generally issue parameterised calls to stored procs, rather than parameterised DML. So I cannot confirm right now that...

Apart from splitting it into two queries, I can't see any way of simplifying what you have written:

DECLARE @max_rmp_vkey INT = (
           SELECT MAX(rmp_vkey)