Using expressions Mapscript, attribute not found in shapefile

[Gregor Mosheh]

Oh, geez, is this one embarrassing.

>    // grab the points layer and tweak it
>    $points_datasource = $TEMPDIR.'/points_'.$_REQUEST['sessionid'].'.shp';
>    $layer = $map->getLayerByName('points');
>    $layer->set('data',$tracks_datasource);
>    $layer->set('status',MS_ON);

I was loading $tracks_datasource instead of $points_datasource. Naturally,
reading the wrong shapefile was not helping.

As the saying goes:
"User error. Replace user and reboot."

Sorry to both y'all. :)