HTTP Error 500 when trying to execucute INSERT INTO sql query - php

Question

I'm fairly new to php and SQL and just can't figure out the problem. Note that this is a school project, therefore the vulnerability to SQL Injections and saving the blank passwords are nothing to worry about.
After the User filled out the Login-form, he's redirected to this page:
[Some html]
<?php
if(isset($_POST['submit']))
{
ConnectSQL();
}
//Retrieve POSTed Login information
$Username = htmlspecialchars($_POST['RegUsername']);
$Email = htmlspecialchars($_POST['RegEmail']);
$Password = htmlspecialchars($_POST['RegPassword']);
function ConnectSQL() {
// SQL Server Extension Sample Code:
// (ConnectionInfo, obviously it's there in the real file)
$conn = sqlsrv_connect($serverName, $connectionInfo);
// PHP Data Objects(PDO) Sample Code:
try {
$conn = new PDO('sqlsrv:server = tcp:xxx.database.windows.net,1433; Database = userdb', 'arechon', '{NotTheRealPassword}');
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
echo "Successfully connected to SQL Server and DB";
Register();
}
catch (PDOException $e) {
print('Error connecting to SQL Server.');
die(print_r($e));
}
}
function Register($Username, $Email, $Password) {
$regquery = "INSERT INTO dbo.Users (Username, Email, Password) VALUES ('UsernameTest', 'EmailTest', 'PasswordTest')";
$conn->query($regquery);
echo '<script type="text/javascript">window.open("http://xxx.azurewebsites.net/Login.html", "_self");</script>';
The Code always seems to stop at $conn->query($regquery); and doesn't return any error messages. Sometimes it just stops, when I slightly modify the Code (e.g. replacing $conn->query($regquery); with $conn->exec($regquery); or using " instead of ') I get a HTTP500 error.
I found some similiar questions here on stackoverflow as well as on other plattforms, but none of the provided answers could solve this error. Note that I use SQL and NOT MySQL (though it wouldn't be a lot of work to change that if you think that could solve my problem).