Can’t access WordPress functions in file called via Ajax?

I am trying to reload the fresh posts using jQuery. As far as I know, I can’t reload the contents of a div inside the page so I reload a file into that div.

The problem is that my loaded file gives me a Fatal error: Call to undefined function wp_query()

How can I implement functions to a newly created file inside the theme directory to work properly?

This is my jQuery

<script language="JavaScript">
$(function() {
    var SANAjax = function(){

    }
    setInterval(SANAjax, 15000 );
});
</script>

And this is what i have in reloadhomeposts.php (I have deleted the content though)

<?php $recent = new WP_Query("cat=3,4,5&showposts=10"); while($recent->have_posts()) : $recent->the_post();?>
.
.
.
<?php endwhile; ?>

The reason for this error is that your loading the file without loading in the WordPress system and so wp_query() doesnt exist.

Quick fix is to:

include("../../../wp-load.php"); <– guessing at the location of the wordpress file.

at the top of the php file.