I'm assuming you mean you want to know how to load an .flv address into your flash player dynamically, from an outside source like php.
1. Set the address of the .flv as a variable in php ... like
PHP Code:
$video = "www.mywebsite.com/flashvideo.flv";
2. when embedding your flash movie in the web page, append the variable in the address of the flash movie.
Code:
flash/myflashmovie.swf?videoid=video
3. in the flash movie, you will have to write actionscript to load that variable into your movie player
Code:
var videolink = _root.videoid
Code:
mymovie.setMedia(""+videolink+"", FLV);
This is assuming you're using Flash 8 Video Player or above. This is a quick way of doing it, there is a better way that's better codewise but this will get you started thinking in the right direction.