I thought I’d share a bit of code that I run on my local WordPress test install to see when WordPress is contacting another website. I originally wrote this to help debug my oEmbed code, but it’s useful for a wide variety of purposes.

<?php

if ( !defined('DOING_AJAX') )
	add_filter( 'http_request_args', 'debug_http_api', 10, 2 );

function debug_http_api( $r, $url ) {
	echo '<p style="text-align:left">HTTP API was used to fetch <code>' . esc_html( $url ) . '</code></p>';

	return $r;
}

?>

The above code will output something like this any time a HTTP request is made using the HTTP API, but only if the request was not made from an AJAX handling script (as it will break the AJAX response):

HTTP API was used to fetch http://www.google.com/

While I’m using a filter to do this, I’m actually using the filter much like an action as I’m not modifying the passed data but merely using the filter as a place to hook in and catch the URL.