android usando cliente Http assíncrono como fazer upload de um arquivo mulit para o servidor var php?

Presumo que você já tenha o arquivo Uri, portanto, do lado do Android, use esta função em AsyncTask para cada arquivo para fazer upload de vários arquivos

public int uploadFile(String sourceFileUri) {

          String fileName = sourceFileUri;

          HttpURLConnection conn = null;
          DataOutputStream dos = null;  
          String lineEnd = "\r\n";
          String twoHyphens = "--";
          String boundary = "*****";
          int bytesRead, bytesAvailable, bufferSize;
          byte[] buffer;
          int maxBufferSize = 1 * 1024 * 1024; 
          File sourceFile = new File(sourceFileUri); 

          if (!sourceFile.isFile()) {


               Log.e("uploadFile", "Source File not exist :"
                                   +uploadFilePath + "" + uploadFileName);

               runOnUiThread(new Runnable() {
                   public void run() {
                       messageText.setText("Source File not exist :"
                               +uploadFilePath + "" + uploadFileName);
                   }
               }); 

               return 0;

          }
          else
          {
               try { 

                     // open a URL connection to the Servlet
                   FileInputStream fileInputStream = new FileInputStream(sourceFile);
                   URL url = new URL("http://example.com/server.php");

                   // Open a HTTP  connection to  the URL
                   conn = (HttpURLConnection) url.openConnection(); 
                   conn.setDoInput(true); // Allow Inputs
                   conn.setDoOutput(true); // Allow Outputs
                   conn.setUseCaches(false); // Don't use a Cached Copy
                   conn.setRequestMethod("POST");
                   conn.setRequestProperty("Connection", "Keep-Alive");
                   conn.setRequestProperty("ENCTYPE", "multipart/form-data");
                   conn.setRequestProperty("Content-Type", "multipart/form-data;boundary=" + boundary);
                   conn.setRequestProperty("uploaded_file", fileName); 

                   dos = new DataOutputStream(conn.getOutputStream());

                   dos.writeBytes(twoHyphens + boundary + lineEnd); 
                   dos.writeBytes("Content-Disposition: form-data; name="uploaded_file";filename=""
                                             + fileName + """ + lineEnd);

                   dos.writeBytes(lineEnd);

                   // create a buffer of  maximum size
                   bytesAvailable = fileInputStream.available(); 

                   bufferSize = Math.min(bytesAvailable, maxBufferSize);
                   buffer = new byte[bufferSize];

                   // read file and write it into form...
                   bytesRead = fileInputStream.read(buffer, 0, bufferSize);  

                   while (bytesRead > 0) {

                     dos.write(buffer, 0, bufferSize);
                     bytesAvailable = fileInputStream.available();
                     bufferSize = Math.min(bytesAvailable, maxBufferSize);
                     bytesRead = fileInputStream.read(buffer, 0, bufferSize);   

                    }

                   // send multipart form data necesssary after file data...
                   dos.writeBytes(lineEnd);
                   dos.writeBytes(twoHyphens + boundary + twoHyphens + lineEnd);

                   // Responses from the server (code and message)
                   serverResponseCode = conn.getResponseCode();
                   String serverResponseMessage = conn.getResponseMessage();

                   Log.i("uploadFile", "HTTP Response is : "
                           + serverResponseMessage + ": " + serverResponseCode);

                   if(serverResponseCode == 200){

                       runOnUiThread(new Runnable() {
                            public void run() {

                                String msg = "File Upload Completed. : "
                                              +uploadFileName;

                                messageText.setText(msg);
                                Toast.makeText(UploadToServer.this, "File Upload Complete.", 
                                             Toast.LENGTH_SHORT).show();
                            }
                        });                
                   }    

                   //close the streams //
                   fileInputStream.close();
                   dos.flush();
                   dos.close();

              } catch (MalformedURLException ex) {


                  ex.printStackTrace();

                  runOnUiThread(new Runnable() {
                      public void run() {
                          messageText.setText("MalformedURLException Exception : check script url.");
                          Toast.makeText(UploadToServer.this, "MalformedURLException", 
                                                              Toast.LENGTH_SHORT).show();
                      }
                  });

                  Log.e("Upload file to server", "error: " + ex.getMessage(), ex);  
              } catch (Exception e) {


                  e.printStackTrace();

                  runOnUiThread(new Runnable() {
                      public void run() {
                          messageText.setText("Got Exception : see logcat ");
                          Toast.makeText(UploadToServer.this, "Got Exception : see logcat ", 
                                  Toast.LENGTH_SHORT).show();
                      }
                  });
                  Log.e("Upload file to server Exception", "Exception : "
                                                   + e.getMessage(), e);  
              }

              return serverResponseCode; 

           } 
         } 

E do lado do servidor server.php

$file_path = "uploads/";

$file_path = $file_path . basename( $_FILES['uploaded_file']['name']);
if(move_uploaded_file($_FILES['uploaded_file']['tmp_name'], $file_path)) {
    echo "Upload Complete";
} else{
    echo "Upload Fail";
}