Converting not work on Mac php without any message

[asika32764]

Example code:

        $r = exec(
            'pngauant image.png',
            $o,
            $rc
        );

        var_dump($r, $o, $rc);

The image was not converted and returns result code as 11:

string(0) ""
array(0) {
}
int(11)

Using Symfony process will result exception with this message:

The process has been signaled with signal "11".

But running pngauant in terminal is work.