What is wrong with this PDO Delete Query?

I'm stripping this thing down to bear bones and I'm still coming up with an error. What am I doing wrong?

Here's the code:

require_once('../bootstrap.php');

$con=DatabaseFactory::CreateOneCommunityConnection();

//$participantid=$_POST['participantid'];
//$readingtypeid = $_POST['readingtypeid'];

$participantid=1;
$readingtypeid = 1;

//1. DELETE ALL READINGS FROM TBLPARTICIPANTREADING

$DeleteReadings=$con->prepare("delete from tblparticipantreading where participantid = :participantid");
$DeleteReadings=bindParam(':participantid', $participantid, PDO::PARAM_INT); //as soon as I add this line, I get an error that says the page can't be displayed. Why?
//$DeleteReadings->execute();

?>

I've got a good database connection. I can run $DeleteReadings=$con->prepare("delete from tblparticipantreading where participantid = :participantid"); and there are no errors, but as soon as I add $DeleteReadings=bindParam(':participantid', $participantid, PDO::PARAM_INT); , I get a 500 error.

What's wrong with what I'm doing? Where am I blowing it?