Class not writing to database PDO - php

Question

I have a class setup to write some data to a mysql database that doesn't seem to be actually writing the data. I believe the issue lies in the PDO statements somewhere. I double checked the query and the database connection on other scripts on the site and they work fine. Any ideas?
Here is my form:
<?php
$navsection = 'addClass';
$dir = $_SERVER['DOCUMENT_ROOT'] . "/grades/";
// load php-login components
require_once $dir . 'php-login.php';
$classes = new Classes();
// load head file
require_once $dir . 'includes/head.php';
?>
<h1>Add a Class</h1>
<?php
// show negative messages
if ($classes->errors) {
foreach ($classes->errors as $error) {
echo $error;
}
}
// show positive messages
if ($classes->messages) {
foreach ($classes->messages as $message) {
echo $message;
}
}
?>
<br />
<form method='post' action='<?php $siteurl; ?>/grades/pages/addClass.php' name='addClass_form'>
<label for='className'>Class Name:</label>
<input id='className' type='text' name='className' required /><br />
<label for='classProfessor'>Professor's Name:</label>
<input id='classProfessor' type='text' name='classProfessor' /><br />
<label for='classPeriod'>Class Period:</label>
<select id='classPeriod' name='classPeriod'>
<option value='Spring 2014'>Spring 2014</option>
<option value='Fall 2013'>Fall 2013</option>
</select><br />
<label for='classStartDate'>Class Start Date:</label>
<input id='classStartDate' type='date' name='classStartDate' /><br />
<label for='classEndDate'>Class End Date:</label>
<input id='classEndDate' type='date' name='classEndDate' /><br />
<input type='submit' name='addClass' value='Submit' />
</form>
<?php
//load footer file
require_once $dir . 'includes/footer.php';
?>
Here is my class:
<?php
class Classes
{
private $db_connection = null;
public $classAdd_successful = false;
public $classDelete_successful = false;
public $classEdit_successful = false;
public $errors = array();
public $messages = array();
public function __construct()
{
session_start();
if (isset($_POST["addClass"])) {
$this->addNewClass($_POST['className'], $_POST['classProfessor'], $_POST['classPeriod'], $_POST['classStartDate'], $_POST['classEndDate']);
}
}
private function databaseConnection()
{
if ($this->db_connection != null) {
return true;
} else {
try {
$this->db_connection = new PDO('mysql:host='.DB_HOST.';dbname='.DB_NAME, DB_USER, DB_PASS);
return true;
} catch (PDOException $e) {
$this->errors[] = "Database error";
return false;
}
}
}
private function addNewClass ($className, $classProfessor, $classPeriod, $classStart, $classEnd)
{
if(empty($className)) {
$this->errors[] = "Please enter a class name.";
} elseif(empty($classProfessor)) {
$this->errors[] = "Please enter a class professor.";
} elseif(empty($classPeriod)) {
$this->errors[] = "Please select a class period";
} elseif(empty($classStart)) {
$this->errors[] = "Please enter a class start date.";
} elseif(empty($classEnd)) {
$this->errors[] = "Please enter a class end date.";
}
if ($this->databaseConnection() == true) {
//Write data to database
$query_new_class_insert = $this->db_connection->prepare('INSERT INTO classes (class_name, user_id, professor_name, class_start, class_end, school_period) VALUES(:className, :userID, :professorName, :classStart, :classEnd, :schoolPeriod)');
$query_new_class_insert->bindvalue(':className', $className, PDO::PARAM_STR);
$query_new_class_insert->bindvalue(':userID', $_SESSION['user_id'], PDO::PARAM_INT);
$query_new_class_insert->bindvalue(':professorName', $classProfessor, PDO::PARAM_STR);
$query_new_class_insert->bindvalue(':classStart', $classStart, PDO::PARAM_STR);
$query_new_class_insert->bindvalue(':classEnd', $classEnd, PDO::PARAM_STR);
$query_new_class_insert->bindvalue(':schoolPeriod', $schoolPeriod, PDO::PARAM_STR);
$query_new_class_insert->execute();
$this->classAdd_successful = true;
} else {
$this->errors[] = "Database write error";
}
}
}
?>