如何将php变量转换为mp3

Question

我正在尝试使用ffmpeg将mp4转换为mp3。

我想从用户上传mp4文件，然后将其转换为mp3

if(isset($_POST['uploadimage'])){
  $file_name = $_FILES['image']['name'];
  $file_size = $_FILES['image']['size'];
  $file_tmp = $_FILES['image']['tmp_name'];
  $file_type = $_FILES['image']['type'];
  $file_ext=strtolower(@end(explode('.',$_FILES['image']['name'])));

  $extensions= array("mp4");

  if(in_array($file_ext,$extensions)=== false){
     $errors[]="extension not allowed, please choose a JPEG file.";
   }
  else if($file_size > 2097100) {
     $errors[]="larger size";
     }
 else if(empty($errors)==true) {
     move_uploaded_file($file_tmp,"input/".$file_name);
     echo"moved";
     $a= "input/".$file_name;
     echo $input_file;
     $b= "output/audio2.mp3";
     exec("ffmpeg.exe");
      exec("ffmpeg -i $a -vn $b", $output, $return);
      if (!$return) {
      echo " Success";
      } 
     else {
     echo " <br>failed";
     }
     }
     } 
     <form method="post" enctype="multipart/form-data">
     <input type='file' name="image" />
     <input type="submit" name="uploadimage" value="Submit">
     </form>

会话未创建 以上是我遇到的错误。