I have a open() command in my Python script which opens the file data.json in my Apache serve running on Raspberry Pi in w mode. This script in turn is run by PHP using the shell_exec command. When the script is run alone, the Python code works. However, it does not function when run by PHP. Does anyone have any idea why this is happening, or is more information needed? Thank you in advance for your help!
Thank you to user2693053 for solving this problem:

Sounds like a privilege issue, although I am not experienced enough
  with calling Python from php to know whether that could be an issue
  here.

and yes, it is a privilege issue! I fixed the problem by doing the sudo chmod 777 * command in the /var/www/html directory.
Python open() function not working when script executed by shell_exec() in PHP