为什么在使用exec功能的php中从avi到mp4的视频转换不起作用？

i tried to convert any video type to mp4 on the server because the html video tag dose not support any type except mp4. so i used exec function to convert the video on the server but it is not working. note : exec function is not in the disabled function in php.ini

my code :

    <?php
      if(isset($_POST['submit']))
      {
      $vidname=$_FILES["vid"]["name"];
      $videotype=$_FILES["vid"]["type"];
      $time = time();
      $vidname2=$time.$_FILES["vid"]["name"];

      move_uploaded_file($_FILES["vid"]["tmp_name"], 'videos/' . $time.$_FILES["vid"]["name"]);
      if($videotype!="video/mp4")
      {
          exec("ffmpeg -i videos/$vidname2 -an videos/$vidname2.mp4"); // Convert .avi to mp4
          unlink("videos/$vidname2");
      }
    }
    else
    {
        ?>
            <form method="post" action="" enctype="multipart/form-data">
                <input type="file" name="vid">
                <input type="submit" name="submit" value="upload">
            </form>
        <?php
    }
?>