You can't read SQL data directly into actionscript.
Easiest way is to make a PHP script to produce the data in XML form and get actionscript to read that from your PHP XML-formatted page.
Google will give you a ton of links about this kind of thing. gotoandlearn.com also has tutorials.
You can use the data -> connect to php in FlashBuilder and generate a php sample file from your mysql db and use that as the service in your flex project.