Problem with using AJAX

[MP3]

Calorie, I checked many topics about AJAX but I didn?t find what I'm looking for.
I have a PHP file, simply I add a sample if condition:

PHP Code:

<?php

require_once('./global.php');

$a = $db->query_first("SELECT bb FROM " . TABLE_PREFIX . "aa WHERE `id`='1'");


if ("$a"=="3")
{
        $play = "<embed src=\"anything.wav\" autostart=true hidden=true></embed>";
}
else
{
        $play="";
}
?>

Now I want to insert the AJAX to my forum header to check the PHP file every 5 min when the condition exist then a small popup will show up without manually refresh. Can you help me?