phpBB 3.2.4 and PHP 7.3 issue

hi,

I was running phpBB 3.2.4 with QI 1.3.3 on MacOs with Apache/2.4.37 (Unix) PHP/7.2.12
I then upgraded my local homebrew php version to 7.3 and it seems phpBB does have trouble. On the UCP page i got :

[phpBB Debug] PHP Notice: in file [ROOT]/includes/functions.php on line 2525: compact(): Undefined variable: username}}
[phpBB Debug] PHP Notice: in file [ROOT]/includes/functions.php on line 2525: compact(): Undefined variable: autologin

I switched back to php 7.2.12 and the issue went away.