I have a Prepared Statement that is supposed to take a list of Message ID's and update several records all at once using a single UPDATE query.

Here is an example of what the actual SQL might look like...
UPDATE private_msg_recipient
SET read_on=NULL,
WHERE member_id_to=19
AND message_id IN (52, 51, 49, 39, 38, 10, 8, 6, 5, 2, 1)
When I run this SQL in phpMyAdmin it runs perfectly., however my PHP isn't working as expected?!

Towards the top of my script I have this code...
	echo '$messagesToUpdate = ' . $messagesToUpdate;
...which yields this on the screen...
$messagesToUpdate = 52, 49, 38

The problem is that when I run my script, my Prepared Statement is only updating the *first* value in $messagesToUpdate...

Here is a snippet of my code...
PHP Code:
    // Build query.
$q1 "UPDATE private_msg_recipient
            SET read_on = NULL,
                updated_on = NOW()
            WHERE member_id_to = ?
            AND message_id IN (?)"

// Prepare statement.
$stmt1 mysqli_prepare($dbc$q1);

// Bind variables to query.

// Execute query.

// Verify Update.
if (mysqli_stmt_affected_rows($stmt1)==1){
// Update Succeeded.

// Update Failed. 

What am I doing wrong here??

(Originally I had 'ii' in my mysqli_stmt_bind_param statement, and I thought that was the issue, but even with 'is' things still aren't working?!)