Debug and dump a WordPress cURL Request and it's response during a WP HTTP API call.

curl_dump.php

<?php
/**
 * Plugin Name: Dump WP HTTP API cURL Request & Response
 * Author:      Franz Josef Kaiser
 */
add_action( 'plugins_loaded', array( 'WPSE81791_cURL', 'init' ) );
class WPSE81791_cURL
{
	protected static $instance;

	public $time_out;
	public $redirections;
	public $version;
	public $is_ssl;

	public static $dump = array();

	public static function init()
	{
		if (
			defined( 'XMLRPC_REQUEST' )
			OR defined( 'DOING_AJAX' )
			OR defined( 'IFRAME_REQUEST' )
		)
			return;

		null === self::$instance AND self::$instance = new self;
		return self::$instance;
	}

	public function __construct()
	{
		// Retrieve filtered vars
		add_filter( 'http_request_timeout', array( $this, 'http_request_timeout' ), 999 );
		add_filter( 'http_request_version', array( $this, 'http_request_version' ), 999 );
		add_filter(
			'http_request_redirection_count',
			array( $this, 'http_request_redirection_count' ),
			999
		);
		add_filter( 'https_ssl_verify', array( $this, 'https_ssl_verify' ), 999 );

		add_action( 'admin_bar_menu', array( $this, 'add_node' ), 120 );

		add_action( 'http_api_curl', array( $this, 'dump_curl' ) );
		add_action( 'shutdown', array( $this, 'do_dump' ) );
	}

	public function add_node()
	{
		is_admin_bar_showing() AND $GLOBALS['wp_admin_bar']->add_node( array(
			'id'     => get_class( $this ),
			'parent' => false,
			'title'  => 'cURL Debug',
			'href'   => '#',
			'meta'   => array(
				'class' => 'foo',
				'html'  => '',
			),
		) );
	}

	/**
	 * Debug the response in the middle.
	 * Catches the cURL object during the request.
	 * @param  cURL $handle
	 * @return void
	 */
	public function dump_curl( &$handle )
	{
		curl_setopt_array( $handle, array(
			CURLINFO_HEADER_OUT    => true,
			CURLOPT_HEADER         => true,
			CURLOPT_HEADERFUNCTION => array( $this, 'dump_curl_buffer_cb' ),
			CURLOPT_WRITEFUNCTION  => array( $this, 'dump_curl_buffer_cb' ),
			CURLOPT_RETURNTRANSFER => true,
			CURLOPT_CERTINFO       => true,
			CURLOPT_FOLLOWLOCATION => true,
			#CURLOPT_NOPROGRESS     => true,
			#CURLOPT_CONNECTTIMEOUT => $this->time_out,
			#CURLOPT_TIMEOUT        => $this->time_out,
			#CURLOPT_HTTP_VERSION   => $this->version,
			#CURLOPT_MAXREDIRS      => $this->redirections,
			#CURLOPT_SSL_VERIFYHOST => $this->is_ssl,
		) );
		curl_exec( $handle );
		$data = sprintf(
			'%s<br />%s',
			curl_getinfo( $handle, CURLINFO_HEADER_OUT ),
			$this->dump_curl_buffer_cb( null )
		);
		0 != curl_errno( $ch ) AND $data .= sprintf(
			'<br />Error Code: %s<br />Error Message: %s',
			curl_errno( $handle ),
			curl_error( $handle )
		);
		$this->add_dump( $data );
	}

	public function http_request_timeout( $sec )
	{
		$this->time_out = $sec;
		return $sec;
	}

	public function http_request_redirection_count( $times )
	{
		$this->redirections = $times;
		return $times;
	}

	public function http_request_version( $version )
	{
		$this->version = $version;
		return $version;
	}

	public function https_ssl_verify( $is_ssl )
	{
		$this->is_ssl = $is_ssl;
		return $is_ssl;
	}

	/**
	 * Callback for cURL dump method
	 * @param  object $curl
	 * @param  null   $data
	 * @return int
	 */
	public function dump_curl_buffer_cb( $curl, $data = null )
	{
		static $buffer = '';
		if ( is_null( $curl ) )
		{
			$r = $buffer;
			$buffer = '';
			return $r;
		}
		$buffer .= $data;
		return strlen( $data );
	}

	/**
	 * Adds data to the static data stack
	 * @param
	 * @return void
	 */
	public function add_dump( $data )
	{
		self::$dump[] = $data;
	}

	/**
	 * Dumps the data stack for debug
	 * @param
	 * @return void
	 */
	public function do_dump()
	{
		! empty( self::$dump ) AND printf(
			'<pre>%s</pre>',
			var_export( implode( "<br />", self :: $dump ), true )
		);
	}
}