[Hexology]

Well I'm very new to php and have been trying to learn what I stated in the above title ^, I need to figure out how to write to a php via link, example would be "www.message.com/write.php?message=hi this is a test", but every time I write to it it'd over write the existing text in the text file. Does anyone know how I would go about doing this?

Also if you know how to do this without overwriting it, and just adding the next entry to a new line, that'd be awesome!