key request;
default
{
    state_entry()
    {
        //I Would Not Recommend Calling This Function In The State Entry
        //This Is Only For Communication Method Demonstrations.
        
        //When passing space characters over HTTPRequest, it's a good idea
        //to use llEscapeURL to make all spaces into %20, etc. This preserves them.
        string message = llEscapeURL("This is my neat message.");
        request = llHTTPRequest("http://www.your-webpage.com/test-file.php", [HTTP_METHOD, "GET"], "message=" + message);
    }
    http_response(key indentifier, integer status, list metadata, string message)
    {
        if(indentifier == request) //This is the return data from our llHTTPRequest() function in state_entry()
        {
            llSay(0, message);
        }
    }
}

//And the PHP looks like this:
<?php
if(isset($_SERVER['message']))
{
     // You can do something with the received message.
     $message = $_SERVER['message'];

     // Now you reply with 'Received!'
     echo "Received!";
}
?>