MYSQL AES_ENCRYPT issues - php

Question

I have a simple application that needs to store sensitive data in a Wordpress installation.
The data comes from the user via a Form, and up until now, I have been storing the data using $wpdb->insert, however as we get closer to launching this project, I want to make sure that sensitive user information is encrypted, and only decrypted when read.
I have no clue how to make this work using the $wpdb class, so I resorted to using mysqli for this component. So far, I run the $wpdb->insert statement to insert all the normal data, then open a new connection to try and insert the sensitive data.
$id = $wpdb->insert_id;
$servername = 'localhost';
$username = 'root';
$password = 'root';
$db = 'wp_database';
$conn = new mysqli( $servername, $username, $password, $db );
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
// Fetch Data and store it
$sud = $form['sens_user_data'];
$key = 'mysecretkey';
$tbl = $wpdb->prefix . 'sensitive';
$sql = "UPDATE $tbl SET sens_user_data = AES_ENCRYPT(?, ?) WHERE id = ?";
if ( $stmt = $conn->prepare($sql) ) {
$stmt->bind_param('ssi', $sud, $key, $id);
$stmt->execute();
} else {
$error = $conn->errno . ' ' . $conn->error;
ChromePhp::log($error);
}
$conn->close();
This appears to work correctly, except all the values always encode to a single character, and then decode to NULL
What am I doing wrong?