Well hi there!
I doesn't understand exactly what securety you mean so I trie to explain possible attacks in general:
the code:
A attacker would be able to extract the URL out of the .EXE file by a decompiler so he would be able to see the "hidden" secret PHP function. You could use a runtime packer(UPX) or other obsurcefactors to scramble the .EXE file but if a cracker wan't to break it he'll do it. The better sollution would be to implement a own crypt function and to add a lot of garbage function so tracing the programm would be difficult.
the request:
Nevertheless the connected server could be analysed by using the
netstat -a command or tools from
www.sysinternals.com. So attackers might see the server. If someone uses a sniffer app he would be able to analyse the complete traffic inluding the url and submitted values. A FW would help him to interrupt the connection. With the help of a proxy attackers might analyse the data, too but furthermore they could inject/change it. This is called man-in-the-middle attack.
To protect your transmissions you should use https sockets but I have no knowledge about it but i think it's just a little bit more difficult(signed certificates,...).
I doesn't know your intension but to register a shareware and so on it's defineitly a bad idea
