What resources or PHP documentation can be referenced to troubleshoot issues with executing external applications using PHP?

When troubleshooting issues with executing external applications using PHP, the PHP documentation on the `exec()` function and the `shell_exec()` function can be valuable resources. Additionally, checking the PHP error logs and the system logs for any relevant error messages can help pinpoint the issue.

$output = shell_exec('your_external_application_command_here');
echo $output;