PHP 5.6. I'm aware that Zen Cart is supposed to now wok up to 7.1 but when I tried upgrading PHP it broke the site and the homepage wouldn't load. All I got was a 404 error. So I downgraded it back to 5.6

This is the error in the debug files.

Code:
PHP Parse error:  syntax error, unexpected '?' in /home/MYWEBSITEDIRECTORY/public_html/MYADMIN/includes/classes/class.CeonURIMappingInstallationCheck.php on line 2173