Android HTTP Request Zero Response

I am trying to make a server request in a php script that will check if the user exists in the database. Currently, I just want to make sure that I get some kind of answer. I try to display the value responseStringwhen the user presses the login button, but each time returns as null. Does anyone know why?

This is my MainActivity

public class MainActivity extends Activity {

EditText username;
EditText password;
Button loginBtn;
LinearLayout loginform;
String passwordDetail;
String usernameDetail;
String url = "http://www.mysite.com/example/checklogin.php";

 String responseString = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //Hide the Action Bar
    ActionBar ab;
    ab = this.getActionBar();
    ab.hide();

    //Get references to XML
    username = (EditText)findViewById(R.id.username);
    password = (EditText)findViewById(R.id.password);
    loginBtn = (Button)findViewById(R.id.loginBtn);
    loginform = (LinearLayout)findViewById(R.id.loginform);

    //Animation
    final AlphaAnimation fadeIn = new AlphaAnimation(0.0f , 1.0f ); 
    AlphaAnimation fadeOut = new AlphaAnimation( 1.0f , 0.0f ) ; 
    fadeIn.setDuration(1200);
    fadeIn.setFillAfter(true);
    fadeOut.setDuration(1200);
    fadeOut.setFillAfter(true);
    fadeOut.setStartOffset(4200+fadeIn.getStartOffset());

    //Run thread after 2 seconds to start Animation
    Handler handler = new Handler();
    handler.postDelayed(new Runnable(){

        public void run() {
            //display login form
            loginform.startAnimation(fadeIn);
            loginBtn.setOnClickListener(new View.OnClickListener() {
                public void onClick(View v) {
                    //display();
                    Toast.makeText(getApplicationContext(), "Checking login details...", Toast.LENGTH_SHORT).show();
                    if(checkLoginDetails()){
                        //OPENS NEW ACTIVITY
                        //Close splash screen
                        //finish();
                        //start home screen 
                        Intent intent = new Intent(v.getContext(), SectionsActivity.class);
                        //startActivity(intent);
                        //creates fade in animation between two activities
                        overridePendingTransition(R.anim.fade_in, R.anim.splash_fade_out);
                        Toast.makeText(getApplicationContext(), "Login Successful" + responseString, Toast.LENGTH_SHORT).show();

                    }
                    else{
                        Toast.makeText(getApplicationContext(), "Login Unsuccessful", Toast.LENGTH_SHORT).show();

                    }
                }
            });

        }

    }, 2000);
}

//Check the login details before proceeding.
public boolean checkLoginDetails(){
    usernameDetail = username.getText().toString();
    passwordDetail = password.getText().toString();
    new RequestTask().execute(url, usernameDetail, passwordDetail);
    return true;
}

This is the php script I am requesting. Right now I have hardcoded data that I know to exist in db and just want to focus on returning the response to say that the user exists.

    <?php
mysql_connect("xxx.xxx.xxx.xxx", "username", "password") or die("Couldn't select database.");
mysql_select_db("databasename") or die("Couldn't select database.");

//$username = $_POST['username'];
//$password = $_POST['password'];

$pwdMD5 = md5(123);

$sql = "SELECT * FROM membership WHERE Username = 'user1' AND Password = '$pwdMD5' ";
$result = mysql_query($sql) or die(mysql_error());
$numrows = mysql_num_rows($result);
if($numrows > 0)
   {
    echo 'user found';
    return true;
   }
else
   {
    echo 'user not found';
    return false;

}
   ?>

This is mine AsyncTask.

 class RequestTask extends AsyncTask<String, String, String>{

    @Override
    protected String doInBackground(String... uri) {
        HttpClient httpclient = new DefaultHttpClient();
        HttpResponse response;
        responseString = null;
        try {
            response = httpclient.execute(new HttpPost(uri[0]));
            StatusLine statusLine = response.getStatusLine();
            if(statusLine.getStatusCode() == HttpStatus.SC_OK){
                ByteArrayOutputStream out = new ByteArrayOutputStream();
                response.getEntity().writeTo(out);
                out.close();
                responseString = out.toString();
            } else{
                //Closes the connection.
                response.getEntity().getContent().close();
                throw new IOException(statusLine.getReasonPhrase());
            }
        } catch (ClientProtocolException e) {
            //TODO Handle problems..
        } catch (IOException e) {
            //TODO Handle problems..
        }
        return responseString;
    }