[PowerPC603]

You can open the .amx in a hex-editor and read the login data for MySQL if they're defined using #define.
I opened mine and I can clearly see my IP address, username, password and database to which my script connects to MySQL.
It's not plain text though, as every character has some character between them (looks like the euro-sign).
http://users.telenet.be/vge/downloads/AMXfile.jpg

If you look closely, you'll see this text (marked in red), see the right side of the program (every character has Ђ between them):
- PPC support filterscript initializing
- 127.0.0.1
- PowerPC603
- ppctrucking
- SomePass
- Connecting to MySQL was successful
- ERROR: connecting to MySQL failed, shutting down server...

So the amx is not entirely safe for storing sensitive data.
Of course, the password was changed to SomePass for showing this data, it's not my real password.

You could of course create a function that creates the credentials using some algorithm, but it may get complex.