What are the common reasons for receiving a "command not found" error when using shell_exec in PHP?

The "command not found" error in PHP's shell_exec function typically occurs when the command being executed is not recognized by the system. This could be due to the command not being installed, the command not being in the system's PATH variable, or the user running the PHP script not having the necessary permissions to execute the command. To solve this issue, you can provide the full path to the command being executed or adjust the PATH variable to include the directory where the command is located.

$output = shell_exec('/full/path/to/command arguments');
echo $output;