Ubuntu – PHP shows up as plain text after upgrade to 18.04

I've just upgraded to Ubuntu 18.04. Now apache serves my website's php as plaintext php. I've already tried:
Why are my PHP files showing as plain text? (on Stack Overflow) and PHP 7 and apache displays plain text and cannot be restarted. The installed php packages are:

php              php7.2-cli       php7.2-fpm       php7.2-mbstring  php7.2-pgsql     php7.2-sqlite3   php7.2-zip       php-curl         php-igbinary     php-pgsql        php-xml
php7.2           php7.2-common    php7.2-gd        php7.2-mysql     php7.2-readline  php7.2-xml       php-bz2          php-fpm          php-mbstring     php-soap         php-xmlrpc
php7.2-bz2       php7.2-curl      php7.2-json      php7.2-opcache   php7.2-soap      php7.2-xmlrpc    php-common       php-gd           php-mysql        php-sqlite3      php-zip

One of the websites in question can be seen here.

The apache error.log:

[Thu Apr 26 16:02:12.740886 2018] [mpm_prefork:notice] [pid 6504] AH00169: caught SIGTERM, shutting down

[Thu Apr 26 16:02:13.001806 2018] [mpm_prefork:notice] [pid 7704] AH00163: Apache/2.4.29 (Ubuntu) OpenSSL/1.1.0g configured -- resuming normal operations

[Thu Apr 26 16:02:13.001900 2018] [core:notice] [pid 7704] AH00094: Command line: '/usr/sbin/apache2'

And the access.log:

<MYIP> - - [26/Apr/2018:16:02:40 +0200] "-" 408 3435 "-" "-"

<MYIP> - - [26/Apr/2018:16:02:40 +0200] "-" 408 3435 "-" "-"

<MYIP> - - [26/Apr/2018:16:02:40 +0200] "-" 408 3435 "-" "-"