You can’t use sudo from a PHP script. Apache is running from an user (www-data generaly), so edit this file : /etc/sudoers
Then add this line :
www-data ALL=(ALL) NOPASSWD:ALL
Care ! this will authorize all functions to be called by a PHP script, you can adapt changing “ALL” by your script or Python command.
Then precise your user in your exec command :
<?php
exec('sudo -u www-data python /usr/lib/cgi-bin/script.py')