Getting "500 Internal Server Error" while uploading Image to server.

I am facing problem while uploading screenshot to server. I am using WWWForm to upload it from unity. It works perfect on localhost. Also i can upload image to the server from web (from html page). But can not upload from unity.

Here is my C# code,

 void Start()
 {
     StartCoroutine(Create_File_Bytes());
 }

 IEnumerator Create_File_Bytes()
 {
     yield return new WaitForEndOfFrame();

     Texture2D Tex = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);
     Tex.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);

     Tex.Apply();

     byte[] file_bytes = Tex.EncodeToPNG();
     Destroy (Tex);

     WWWForm form = new WWWForm();
     form.AddBinaryData("selected_image", file_bytes, "sample1.jpg", "image/png");

     WWW w = new WWW("http://<domain_name>/Upload_File.php", form);

     yield return w;

     if (w.error != null)
     {
         debug.log("Upload Error : " + w.error + " \n Server Says : " + w.text);
     }
     else
     {
         debug.log("Finished Uploading Screenshot : " + w.text);
     }

And this is Upload_File.php code,

if(isset($_FILES['selected_image'])) { if (file_exists("Uploaded_Files/" . $_FILES["selected_image"]["name"])) { echo $_FILES["selected_image"]["name"] . " already exists. "; } else { move_uploaded_file($_FILES["selected_image"]["tmp_name"], "Uploaded_Files/" . $_FILES["selected_image"]["name"]); echo "Stored in: " . "Uploaded Files/" . $_FILES["selected_image"]["name"]; } } else { echo "File is not selected"; }