C# return string from PHP function

I have face the exact problem like you.
Using PHP script to connect SilverLight to MySQL is my first approach.

But later, I had discovered a more convenient/easier way, by using C# Web Services.

I have written an article explaining the steps in details with illustrations.
>> Connecting MySQL From SilverLight With Web Services[^]

Simple explanation:
1. You need to create a C# web services.
2. Configure the web service access permission
(by default, web service only allow to be access by localhost)
3. At SilverLight application, add a service reference of the WebService that you have created.

There, at your SilverLight, you access MySQL just as executing a method.

Example:

code at SilverLight

public partial class MainPage : UserControl
{
    ServiceReference1.WebService1SoapClient myService;
 
    public MainPage()
    {
        InitializeComponent();
        myService = new ServiceReference1.WebService1SoapClient();
        myService.ExecuteScalarCompleted += myService_ExecuteScalarCompleted;
    }
  
    void myService_ExecuteScalarCompleted(object sender, ServiceReference1.ExecuteScalarCompletedEventArgs e)
    {
        MessageBox.Show(e.Result);
    }
 
    private void button_ExecuteScalar_Click(object sender, RoutedEventArgs e)
    {
        myService.ExecuteScalarAsync(textBox1.Text);
    }
}

code at Web Service

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.Services;
using MySql.Data.MySqlClient;
using System.Data;

namespace MyWebService
{
    /// <summary>
    /// Summary description for WebService1
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    public class WebService1 : System.Web.Services.WebService
    {
        [WebMethod]
        public string ExecuteScalar(string sql)
        {
            string result = "";
            string constr = "server=localhost;user=root;pwd=1234;database=member;";
            using (MySqlConnection conn = new MySqlConnection(constr))
            {
                MySqlCommand cmd = new MySqlCommand();
                cmd.Connection = conn;
                conn.Open();

                cmd.CommandText = sql;
                result = cmd.ExecuteScalar() + "";

                conn.Close();
            }

            return result;
        }
    }
}

The above gives you an idea how SilverLight communicate MySQL through WebServices.

There are some details that I didn't mentioned above.

You have to search google on below details:
1. How to create a C# Web Service.
2. How to add a Web Service reference at SilverLight

From the above example, you can see that SilverLight is accessing the Web Service with this:

ServiceReference1.WebService1SoapClient myService;

ServiceReference1.WebService1SoapClient is an class automatically generated by Visual Studio. We only need to locate the Web Services location (example: http://www.myweb.com/WebServices.asmx), then data binding will be automatically performed.

Web Services has a file extension of ".asmx".