/* Automatically change links every week
 * Drop this into functions.php of your theme
 */
function tbm_display_resource() {
	$today = strtotime( date('m/d/y') );
	$wk1 = strtotime( '10/01/12' );
	$wk2 = strtotime( '10/07/12' );
	$wk3 = strtotime( '10/14/12' );
	$wk4 = strtotime( '10/21/12' );
	
	if( ( $today >= $wk1 ) && ( $today < $wk2 ) ) {
		$gt = 'http://url-to-file-for-week-1.pdf';
	} elseif ( ($today >= $wk2) && ($today < $wk3) ) {
		$gt = 'http://url-to-file-for-week-2.pdf';
	} elseif( ($today >= $wk3) && ($today < $wk4) ) {
		$gt = 'http://url-to-file-for-week-3.pdf';
	} elseif( $today >= $wk4 ) {
		$gt = 'http://url-to-file-for-week-4.pdf';
	}

	return '<p style="text-align: center;"><a href="'. $gt .'"><img class="aligncenter size-full wp-image-1439" title="Cool resource" src="http://url-to-image.jpg" alt="" width="247" height="151" /></a></p>';
}
add_shortcode( 'autoresource', 'tbm_display_resource' );