Newer versions of PHP will disable PHP from working in user's public_html directories.

Find which file needs editing: ls -l /etc/apache*/mods-enabled/*php*.conf

It should result in a specific file pointed by the symbolic link (e.g., /etc/apache2/mods-available/php7.4.conf). Inside that file, locate a section that looks like this:

# Running PHP scripts in user directories is disabled by default
#
# To re-enable PHP in user directories comment the following lines
# (from <IfModule ...> to </IfModule>.) Do NOT set it to On as it
# prevents .htaccess files from disabling it.
<IfModule mod_userdir.c>
    <Directory /home/*/public_html>
        php_admin_flag engine Off
    </Directory>
</IfModule>

And comment out the section that turns of PHP.

# Running PHP scripts in user directories is disabled by default
#
# To re-enable PHP in user directories comment the following lines
# (from <IfModule ...> to </IfModule>.) Do NOT set it to On as it
# prevents .htaccess files from disabling it.
# <IfModule mod_userdir.c>
#    <Directory /home/*/public_html>
#        php_admin_flag engine Off
#    </Directory>
#</IfModule>

Then, restart the web server:

sudo apachectl restart

Looks like this happened to me a while back in late 2017, too.