Reloading A Number Without Refreshing The Page?

I'm trying to create a feature similar to facebooks "like" system.

I have a page with peoples posts in, and when someone clicks on "like", I want to first run the query to insert a new like into the database, then reload just the number of likes, without refreshing the entire page.

I know this way is harder but I want it to work without reloading everything. Here's what I have so far:

function for like (reverse engineered, might be completely wrong..)

function like(){
$('#postoptions').load('php_files/likecount.php');
}
postoptions.php

[Code]....

I know that at the moment there is no query to add an entry to the database when clicking on the like link, but I'm not too sure how to put it. I tried putting this:

$addLike = "INSERT INTOlike (username, postid) VALUES ('$username','$postid')";
$run = mysql_query($addLike) or die(mysql_error());

but because the posts are loaded with a while loop, clicking like on one made me like all of them on that page.

I though reloading the "likecount.php" page would reload the count for the page?