Yes, same here, I use the web interface on my Synology and learn as I go. I ran phpinfo.php and both /www and /web are using PHP Version 5.6.40.
PHP 5.6, 7.0, 7.2, 7.3, and 7.4 are all installed on my Synology. I'm running the current version of DSM: 7.0.1-42218 Update 2.
See attached screen capture from my Default server portal settings - are you suggesting I just change the dropdown Default Profile from 5.6 to 7.4 and see what happens?