exec() or system() on doxygen command returns me “Exiting”

[Epimbestknow]

Itry to run this script\[code\] public function execDoxygen($doxyFile) {\[code\] $command = "doxygen $doxyFile"; exec($command, $output); return $output;}\[/code\]\[/code\]and the outputs is "Exiting..."if I run it separetely in a terminal with the same file it works well.If I run exec("doxygen --help") it works correctly.Why does it not work with a variable?