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.
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.
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
Of course, an alternative to this is to run apache as ubuntu (edit /etc/apache2/envvars), which will make all the above unnecessary assuming the situation enables it.