PHP script returns the raw code rather than echoed variable in Android

Question

I'm implementing an Android-MySQL interaction with PHP scripts. The script works fine when I was running it on my laptop.

When I was trying to handle the return value from the PHP scripts (returned by echo) in my Android application, it turns out the whole raw PHP script was returned.

I was using Genymotion as the emulator and I did set the url as 10.0.3.2 in my code.

While I was trying to visit the PHP page using Genymotion (10.0.3.2/test.php), the browser load the raw script as well.

Although I think its more like a port/configuration problem, below are my PHP scrips and Android code snippet.

PHP:

<?php

$db_name = "demo";
$mysql_username = "root";
$mysql_password = "password";
$server_name = "127.0.0.1";

$sql_query = "select * from table1;";

// establish connection to mysql database
$con = mysqli_connect($server_name, $mysql_username, $mysql_password, 
$db_name);

$result = mysqli_query($con, $sql_query);

$return_arr = array();

while ($row = mysqli_fetch_array($result))
{
    $return_arr[] = array(
        'id' => $row['id'],
        'size' => $row['size'],
        'material' => $row['material'],
        'date' => $row['date']
    );
}

header('Content-Type: application/json');
echo json_encode(array("server_response"=>$return_arr));

mysqli_close($con);

?>

Android:

System.out.println("nooo!");

        StringRequest stringRequest = new StringRequest(Request.Method.POST, json_url, new Response.Listener<String>() {
            @Override
            public void onResponse(String response) {
                System.out.println(response);
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
            }
        });

        queueVolley.add(stringRequest);

The question is how make the PHP script return the correct data in Android? It was running on a MacBook Pro with Mojave OS.

Thank you!