The problem
Python script invoked from PHP via shell_exec and runs fine when PHP invoked from command line but fails when PHP triggered by browser access.

Reason
PHP, when triggered by a browser access is invoked by web server with the user www-data, while from the command line it is run as user ubuntu.

Attempting to see what happens when running PHP from the command line as user www-data would help to understand why running the script fails

One method to run php as www-data from the command line is to enable a shell for www-data user. This is done by modifying /etc/passwd so that user www-data has a shell (change the existing /usr/sbin/nologin or whatever to /bin/bash or something similar) and then sudo su www-data and try to run the python script again (see this reply for details).

Doing the above, quickly showed that the one of the imports in the python script fails when running it under www-data.
Comparing python3 -m site when running under user www-data vs. when running under ubuntu showed there is a difference in the module search paths.

Adding the missing path found for user ubuntu to user www-data via sys.path.insert was not scalable, nor possible (since the ubuntu user path is inaccessible to the www-data user), so the best way was to install the python modules (in my case, imagehash) in a way that will be accessible to the www-data user

The solution, found here illustrated how this is done

sudo mkdir /var/www/.local
sudo mkdir /var/www/.cache
sudo chown www-data.www-data /var/www/.local
sudo chown www-data.www-data /var/www/.cache
sudo -H -u www-data pip install imagehash

Method #2 ∞

Of course, a simpler alternative to this is to run apache as ubuntu, which will make all the above unnecessary assuming the situation/security requirements enable it, in which case you might want to also change the htdocs directory:

sudo vim /etc/apache2/envvars # change APACHE_RUN_USER and APACHE_RUN_GROUP to ubuntu
cd /etc/apache2/sites-available/
sudo cp 000-default.conf 000-ubuntu.conf
sudo vim 000-ubuntu.conf   # change the path for DocumentRoot
sudo a2dissite 000-default.conf
sudo a2ensite 000-ubuntu.conf

Also,
sudo vim /etc/apache2/apache2.conf

and add the following:

<Directory /home/ubuntu/>
        Options Indexes FollowSymLinks
        AllowOverride None
        Require all granted
</Directory>

Don’t forget to

sudo systemctl restart apache2

or

service apache2 reload