Assuming this example HTML:

<input type=text name=email id=email />

<input type=text name=first_name id=first_name />

<input type=text name=last_name id=last_name />

You could have this javascript:

$(#email).bind(change, function(e){

  $.getJSON(http://yourwebsite.com/lokup.php?email= + $(#email).val(),

        function(data){

          $.each(data, function(i,item){

            if (item.field == first_name) {

              $(#first_name).val(item.value);

            } else if (item.field == last_name) {

              $(#last_name).val(item.value);

            }

          });

        });

});

Then just you have a PHP script (in this case lookup.php) that takes an email in the query string and returns a JSON formatted array back with the values you want to access. This is the part that actually hits the database to look up the values:

<?php

//look up the record based on email and get the firstname and lastname

...



//build the JSON array for return

$json = array(array('field' => 'first_name', 

                    'value' => $firstName), 

              array('field' => 'last_name', 

                    'value' => $last_name));

echo json_encode($json );

?>

You'll want to do other things like sanitize the email input, etc, but should get you going in the right direction.