In order for a browser to play the MP3 file, it will need to download the file into temporary memory. Meaning it can be retrieved using certain browser plugins. Something to be aware of. The usual remedy is to offer low quality and cropped audio clips.
The best resources (and most detailed) are usually the home sites of the technologies. e.g. If you want to learn the most about HTML, the World Wide Web Consortium's website is the best resource. The PHP Group for PHP and the Apache HTTP Server's project site for .htaccess. A .htaccess file is an apache web server file executed at runtime.
Not to leave you stranded in the dark, here's a useful explanation on how to prevent hotlinking: http://www.cyberciti.biz/faq/apache-...eeching-howto/
“The best thing about a boolean is even if you are wrong, you are only off by a bit.”