The Server-Side Routine

 

In our example here, the server-side routine, checkuser.php, is written in PHP.

 

Bear in mind, though, that Ajax is pretty much agnostic about what server-side technology is being used - this routine could be in ASP, JSP or a host of other languages. For this reason, we won't go into too much detail over this code - except that, it uses PHP to interrogate a mySQL database user_table to see whether the column user already contains the username that our visitor has tried to add: 

checkuser.php

// database connection code
$HOST = "my.database.server";
$DATABASE = "foo";
$USER = "bar";
$PASSWORD = "whatever";
$conn = mysql_connect($HOST,$USER,$PASSWORD);
mysql_select_db($DATABASE,$conn);

// check if the user is already in the database
// the username to test is passed in variable uname
$result = mysql_query("SELECT * FROM user_table WHERE user = '$uname'");

// count matching fields
$count = mysql_num_rows($result);

// if there are any, we can't accept the username
if($count > 0)
{  
   echo '0';   // failed
} else {   
   echo '1';   // success!
}

This returned code is checked by our callback routine. You'll recall that we left this empty, but now we can add some code:

function handleHttpResponse() {
  if (http.readyState == 4) {
     if(http.responseText == '1') {
         document.myform.submit();
         } else {
         alert('That username is already taken. Please choose another');
         return false;
        }
    }
}

Now we need to write our form checking routine. We'll use the submit button's onclick() method to call the routine:

The checkform() routine will first check the nick field to make sure it's not empty, then call our Ajax routine to check the user name:

function checkform()
{
if(document.myform.nick.value='')
   {
   alert('Please fill in a Nickname');
   return false;
   }
checkuser();
}

And that's about it. I haven't included a demo here, but please see the tutorial on CAPTCHA, which uses edxactly this technique for form validation.

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