Using an enum from another file as a parameter in PHP - php

Question

I have a php file that handles mailing. The Mail() function requires a case to determine the subject and body of the email that is to be sent. To prevent the caller from giving wrong parameters I am using an enum as shown below:
<?php
enum MailCase
{
case bestOffer;
case notForSale;
}
function Email($product_ean, MailCase $case)
{
// TODO: Product titel in onderwerp.
$to = 'receiver#company.com';
$headers = 'From: sender#company.com' . "\r\n" .
'Reply-To: sender#company.com' . "\r\n" .
'X-Mailer: PHP/' . phpversion();
switch ($case) {
case MailCase::bestOffer:
$subject = 'Lorem ipsum 1';
$message = '
This body is variant 1 for case 1.
PRODUCT: ' . $product_ean;
break;
case MailCase::notForSale:
$subject = 'Lorem ipsum 2';
$message = '
This body is variant 2 for case 2.
PRODUCT: ' . $product_ean;
break;
}
mail($to, $subject, $message, $headers);
}
This php file is required in the main file and is called via require_once("mailer.php");
Then, the following function calls the function that starts the mail procedure:
alertRetailer($product, MailCase::notForSale);
Which executes this:
function alertRetailer($product, MailCase $case)
{
Email($product, $case);
}
When running this program the following parse error is show in the browser:
Parse error: syntax error, unexpected 'MailCase' (T_STRING) in /domain/mailer.php on line 3
As far as I can see the file that provides the mailing functionality and the enum is required the right way and IntelliSense doesn't show an error that the file is not found. I would like to hear you guys' view on my scenario.
EDIT:
PHP version was checked:
Thanks!