为什么图像不是从php中的视频生成的

I have a upload for a video to go to a folder but the upload works and not the generation from the video i have tried echo and i have googled this problem but no results so far if i could get any one to help it would be appreciated.

CODE:

<?php
if(!isset($_SESSION['fname']) && !isset($_SESSION['username']) &&         !isset($_SESSION['email'])) {
  redirect_to("index.php");
} else {
if(isset($_POST['submit'])) {
$allowedExts = array("jpg", "jpeg", "gif", "png", "mp3", "mp4", "wma");
$extension = pathinfo($_FILES['file']['name'], PATHINFO_EXTENSION);

if (in_array($extension, $allowedExts))

  {
  if ($_FILES["file"]["error"] > 0)
    {
    echo "Return Code: " . $_FILES["file"]["error"] . "<br />";
    }
  else
    {

if (file_exists("uploads/" . $_FILES["file"]["name"]))
  {
  echo $_FILES["file"]["name"] . " already exists. ";
  }
else
  {
  move_uploaded_file($_FILES["file"]["tmp_name"],
  "uploads/" . $_FILES["file"]["name"]);        
  $target_path = 'uploads/Sequence 01.mp4';
  $imgPath = 'thumbs/thumb.jpg';
echo exec("ffmpeg  -itsoffset -0 -i uploads/" . $_FILES["file"]["name"] . " -vcodec mjpeg -vframes 0 -an -f rawvideo -s 200x200 thumbs/" . $_FILES["file"]["name"] . ".jpg");
        $name = $_SESSION["username"];
      $sql = "INSERT INTO `videos`(`filename`, `user_uploaded`, `previewfn`) VALUES ('{$_FILES["file"]["name"]}','{$name}','{$_FILES["file"]["name"]}.jpg')";
      $result = mysqli_query($db, $sql);
      }                                                                                                                                               
    }
  }
else                                                                        
  {                                                              
  echo "Invalid file";              
  }

}
}
?>

<form action="upload-video.php" method="post" enctype="multipart/form-data">
<label for="file"><span>Filename:</span></label>
<input type="file" class="btn bg-danger" name="file" id="file" /> 
<br /><br />
<input type="text" class="input" name="title" id="title" />
<input type="submit" name="submit" class="btn" value="Submit" />
</form>

ffmpeg  -itsoffset -0 -i uploads/...

should be:

ffmpeg  -itsoffset 0 -i uploads/...

Not minus before zero