1) finding out files on the server:
$dir=opendir("./thatswhatiwant/")
readdir($dir) //"."
readdir($dir) //".."
do{
$file=$readdir($dir);
echo $file."";
}while($file!="");
or you save all files in a thumb.
2) Selecting and printing the stuff in a textbox is made in JS. Use something like textbox.value+="; "+selected.value
or you use the html-list with multiple-entries (don't get the tag right now) and name it
... name="files[]"
important are the brakets. In php you can then get all of the selected files.
I hope that helps. If not, then, well, just ask again ^^.
Simon