The one way is by using webservice, simply write a webservice method in PHP or any other language . And From your android app by using http client request and response , you can hit the web service method which will return whatever you want.
For PHP You can create a webservice like this. Assuming below we have a php file in the server. And the route of the file is yourdomain.com/api.php
if(isset($_GET['api_call'])){
switch($_GET['api_call']){
case 'userlogin':
//perform your userlogin task here
break;
}
}
Now you can use Volley or Retrofit to send a network request to the above PHP Script and then, actually the php script will handle the database operation.
In this case the PHP script is called a RESTful API.
You can learn all the operation at MySQL from this tutorial. Android MySQL Tutorial to Perform CRUD.