FFmpeg output to textfile causing 500 Internal Server Error in PHP - php

Question

I have a PHP script with two FFmpeg commands. The first command combines a list of .ts files into a single .ts file and logs the output information into a textfile. The second command converts the combined .ts file into an mp4 file and logs the output information into a textfile.
Here's the PHP:
<?php
error_reporting(E_ALL);
ini_set('display_errors', 'On');
$ffmpeg = "/usr/local/bin/ffmpeg";
$vidClips = 'my_vids/vidClipslist.txt';
$combinedFileTs = 'my_vids/combinedFileTs.ts';
$logFileCombine = 'my_vids/logFileCombine.txt';
$logFileConvert = 'my_vids/logFileConvert.txt';
$combinedFileMp4 = 'my_vids/combinedFileMp4.mp4';
shell_exec("$ffmpeg -f concat -safe 0 -i $vidClips -c copy $combinedFileTs 1> $logFileCombine 2>&1");
shell_exec("$ffmpeg -i $combinedFileTs -f mpegts -codec:v mpeg1video -bf 0 -codec:a mp2 -q 12 $combinedFileMp4 1> $logFileConvert 2>&1");
echo "Video finished.";
?>
When I run this, my 500.shtml file message is shown on the page. However, I get no other error message echoed to the page nor are there any errors in my Cpanel error log. I also have a php error log set up, but it doesn't show any errors.
Oddly, both of the FFmpeg commands do what they're supposed to, i.e., the "combinedFileTs.ts", "logFileCombine.txt", "combinedFileMp4.mp4", and "logFileConvert.txt" are all created. It's just that I'm getting the 500 error and the PHP script following the two FFmpeg commands is not being run, i.e., echo "video finished."
When I remove the output directive on the first FFmpeg command, i.e., "1> $logFileCombine 2>&1", everything works. All the files are created, the proceeding PHP script is run, and I don't get the 500 error.
<?php
error_reporting(E_ALL);
ini_set('display_errors', 'On');
$ffmpeg = "/usr/local/bin/ffmpeg";
$vidClips = 'my_vids/vidClipslist.txt';
$combinedFileTs = 'my_vids/combinedFileTs.ts';
$logFileCombine = 'my_vids/logFileCombine.txt';
$logFileConvert = 'my_vids/logFileConvert.txt';
$combinedFileMp4 = 'my_vids/combinedFileMp4.mp4';
shell_exec("$ffmpeg -f concat -safe 0 -i $vidClips -c copy $combinedFileTs");
shell_exec("$ffmpeg -i $combinedFileTs -f mpegts -codec:v mpeg1video -bf 0 -codec:a mp2 -q 12 $combinedFileMp4 1> $logFileConvert 2>&1");
echo "Video finished.";
?>
Also, the first FFmpeg command takes about 2 min to run. Thinking that the problem was due to the PHP script being killed, I increased the max_execution_time to 600 and the memory_limit to 512M in my php.ini file but that did not fix the problem.
I need both FFmpeg commands to produce an output log textfile and I need the proceeding PHP code to run.