The first part is easy:
$feature = file("dat/" . $FORM['whichone'] . ".dat");
Then carry on almost exactly as before (only difference is that arrays don't take a different prefix character in php):
$number = $feature[0];
However, why not improve things while you're at it? Since the lines in the file, and the elements of the array, only have meaning based on the relative position, why not cut out the array entirely:
list(
$number,
$section,
...
$mediadescription2,
$mediadescription3
 ðŸ˜‰ = file( "dat/" . $FORM['whichone'] . ".dat");