[php] PHP code is not being executed, instead code shows on the page

Another possible cause of this problem could be that you are trying to run the script in a "user directory" from the UserDir module. Running PHP scripts in user directories is disabled by default. You will run into this problem if the script is in the public_html directory in your home folder and you are trying to access it from http://localhost/~your_username.

To fix this, open up /etc/apache2/mods-enabled/php7.2.conf. You must comment or delete the tag block at the bottom that reads

<IfModule mod_userdir.c>
    <Directory /home/*/public_html>
        php_admin_flag engine Off
    </Directory>
</IfModule>