I finally got it to work!!!!!

Here's what I did:

1. created a shell script (myfile.sh) with my '/usr/local/bin/ffmpeg -i .......' command
2. put script in a executable directory
3. called the script from php with the full path
exec('/path/to/my/shell/script/myfile.sh');
4. edited the sudoers file using 'visudo' command in terminal
5. added this line: 'apache ALL=NOPASSWD: /path/to/shell/script'
6. commented out 'Defaults requiretty'
7. ':w' to write ':q' to quit visudo

days of struggle and searching have finally come to an end