ffmpeg只在命令行上工作，而不是php

I am using FFmpeg https://github.com/olaferlandsen/ffmpeg-php-class. But when i am running code it is not working. Please help me to solve this issue if anyone know. Below is the code

$FFmpeg = new FFmpeg( "/etc/bin/ffmpeg" );
$FFmpeg->input( '/opt/lampp/htdocs/ffmpeg/1.wav' )->output( '/opt/lampp/htdocs/ffmpeg/ss.mp3' );
$FFmpeg->ready();

use without cunstructor:-

        $FFmpeg = new FFmpeg();
        $FFmpeg->input( '/opt/lampp/htdocs/ffmpeg/1.wav' )->output( '/opt/lampp/htdocs/ffmpeg/ss.mp3' );
        $FFmpeg->ready();