Well you cant really compare Java to PHP can you? PHP is a web based programming language, and though you can run Java apps on a website, it is definitely not "web-based".
As to how to learn PHP quickly, take a simple free script from whatever website that adds, edits and deletes strings, such as a shoutbox or a guestbook, see how the script works and try making your own script out of it.. eg. download a php guestbook and try 'converting' it into say.. a news poster, with comments even.. it aint hard, its incredibly simple, all you need to learn is how to handle a submitted html form with php, and how to add/modify/delete items with php..
I strongly suggest you start out working with mysql straight away, flatfile sucks.