android error org.json.JSONException: Value <!DOCTYPE of type java.lang.String cannot be converted to JSONObject

Finally, I solved my problem using a custom json_decode method in order to clean the json string before decoding it.

function json_clean_decode($json, $assoc = false, $depth = 512, $options = 0) {
    // search and remove comments like /* */ and //
    $json = preg_replace("#(/*([^*]|[rn]|(*+([^*/]|[rn])))**+/)|([st]//.*)|(^//.*)#", '', $json);
    // search and remove all backslashes
    $json = str_replace("\","", $json);

    if(version_compare(phpversion(), '5.4.0', '>=')) {
        $json = json_decode($json, $assoc, $depth, $options);
    }
    elseif(version_compare(phpversion(), '5.3.0', '>=')) {
        $json = json_decode($json, $assoc, $depth);
    }
    else {
        $json = json_decode($json, $assoc);
    }

    return $json;
}

After almost searching for 3+ weeks and n+ hours of frustration, i've finally found an working solution to my problem and sharing it with people so they could benefit from it (as i was blocked from another a/c due to asking too many questions and down votes ) , in order to send your array list to your server we need to use another library and the best fit according to my need is async-http library by loop j and these are the steps to import and use the library in your program :-

1.)import the library into your project by writing this statement in your bulid.gradle(app module):-

compile 'com.loopj.android:android-async-http:1.4.9'

2.)Create the following variables in order to use them in your program:-

 AsyncHttpClient (a client to send data to your server)
    RequestPrarms (Data sent to your server for parsing and further operations )
    String url(Link to your server where actually operations occurs)

3.) Now we use these variables and run the program :-

p

arams.put("OrderSummary", <your array list>);
           httpClient.post(APIURL.API_URL,params, new AsyncHttpResponseHandler() {
               @Override
               public void onSuccess(int statusCode, Header[] headers, byte[] responseBody) {
                   //your operations on success
               }

               @Override
               public void onFailure(int statusCode, Header[] headers, byte[] responseBody, Throwable error) {
                    Toast.makeText(getApplicationContext(),error.toString(),Toast.LENGTH_LONG).show();
               }
           });  

i hope this clears your doubts somewhat i'm still creating my php side in order to insert data in the database