Recently i was working on a project that required a flash media player.  I was trying to integrate JW FLV Media Player using SWFObject.  After losing a few hours of my life trying to pass a variable in a query string to a php script that generated a XML play list, I stumbled upon/discovered this solution.

the JW FLV Media Player documentation currently reads like this

<p id="preview">The player will show in this paragraph</p>
<script type='text/javascript' src='swfobject.js'></script>
<script type='text/javascript'>
var s1 = new SWFObject('player.swf','player','400','300','9');
s1.addParam('allowfullscreen','true');
s1.addParam('allowscriptaccess','always');
s1.addParam('flashvars','file=video.flv');
s1.write('preview');
</script>

here is the script that I had to use to get my id to pass to my php script

<script type='text/javascript' src='swfobject.js'></script>
<script language="javascript" type="text/javascript">

var so = new SWFObject("flvplayer.swf", "player", "300", "250", "9"); // Location of swf file.
so.addParam("quality", "high");
so.addVariable('file', 'product_playlist.xml');
so.addVariable('autostart', 'true');
so.addVariable('screencolor', '0xfffffff'); 
so.addParam("wmode", "transparent"); so.write("player"); 
</script>

turns out using that addParam as stated in the documentation will not work. You have to use the addVariable method for the "file" attribute and pass the file name and any other query string args (pid=1&test=yes) to get it to work