To API Java code with PHP, and connect a Java frontend with a PHP backend that will connect to a database, you just call the PHP file's URL in the code, correct?
URL url = new URL("http://10.0.3.2/MYCODE/app/login.php");
String urlParams = "name="+name+"&password="+password;
HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
httpURLConnection.setDoOutput(true);
OutputStream os = httpURLConnection.getOutputStream();
os.write(urlParams.getBytes());
os.flush();
os.close();