WP::send_headers()

	public function send_headers() {
		$headers = array();
		$status = null;
		$exit_required = false;

		if ( is_user_logged_in() )
			$headers = array_merge($headers, wp_get_nocache_headers());
		if ( ! empty( $this->query_vars['error'] ) ) {
			$status = (int) $this->query_vars['error'];
			if ( 404 === $status ) {
				if ( ! is_user_logged_in() )
					$headers = array_merge($headers, wp_get_nocache_headers());
				$headers['Content-Type'] = get_option('html_type') . '; charset=' . get_option('blog_charset');
			} elseif ( in_array( $status, array( 403, 500, 502, 503 ) ) ) {
				$exit_required = true;
			}
		} elseif ( empty( $this->query_vars['feed'] ) ) {
			$headers['Content-Type'] = get_option('html_type') . '; charset=' . get_option('blog_charset');
		} else {
			// Set the correct content type for feeds
			$type = $this->query_vars['feed'];
			if ( 'feed' == $this->query_vars['feed'] ) {
				$type = get_default_feed();
			}
			$headers['Content-Type'] = feed_content_type( $type ) . '; charset=' . get_option( 'blog_charset' );

			// We're showing a feed, so WP is indeed the only thing that last changed.
			if ( ! empty( $this->query_vars['withcomments'] )
			     || false !== strpos( $this->query_vars['feed'], 'comments-' )
			     || ( empty( $this->query_vars['withoutcomments'] )
			          && ( ! empty( $this->query_vars['p'] )
			               || ! empty( $this->query_vars['name'] )
			               || ! empty( $this->query_vars['page_id'] )
			               || ! empty( $this->query_vars['pagename'] )
			               || ! empty( $this->query_vars['attachment'] )
			               || ! empty( $this->query_vars['attachment_id'] )
			          )
			     )
			) {
				$wp_last_modified = mysql2date( 'D, d M Y H:i:s', get_lastcommentmodified( 'GMT' ), false );
			} else {
				$wp_last_modified = mysql2date( 'D, d M Y H:i:s', get_lastpostmodified( 'GMT' ), false );
			}

			if ( ! $wp_last_modified ) {
				$wp_last_modified = date( 'D, d M Y H:i:s' );
			}

			$wp_last_modified .= ' GMT';

			$wp_etag = '"' . md5($wp_last_modified) . '"';
			$headers['Last-Modified'] = $wp_last_modified;
			$headers['ETag'] = $wp_etag;

			// Support for Conditional GET
			if (isset($_SERVER['HTTP_IF_NONE_MATCH']))
				$client_etag = wp_unslash( $_SERVER['HTTP_IF_NONE_MATCH'] );
			else $client_etag = false;

			$client_last_modified = empty($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? '' : trim($_SERVER['HTTP_IF_MODIFIED_SINCE']);
			// If string is empty, return 0. If not, attempt to parse into a timestamp
			$client_modified_timestamp = $client_last_modified ? strtotime($client_last_modified) : 0;

			// Make a timestamp for our most recent modification...
			$wp_modified_timestamp = strtotime($wp_last_modified);

			if ( ($client_last_modified && $client_etag) ?
					 (($client_modified_timestamp >= $wp_modified_timestamp) && ($client_etag == $wp_etag)) :
					 (($client_modified_timestamp >= $wp_modified_timestamp) || ($client_etag == $wp_etag)) ) {
				$status = 304;
				$exit_required = true;
			}
		}

		/**
		 * Filters the HTTP headers before they're sent to the browser.
		 *
		 * @since WP-2.8.0
		 *
		 * @param array $headers The list of headers to be sent.
		 * @param WP    $this    Current ClassicPress environment instance.
		 */
		$headers = apply_filters( 'wp_headers', $headers, $this );

		if ( ! empty( $status ) )
			status_header( $status );

		// If Last-Modified is set to false, it should not be sent (no-cache situation).
		if ( isset( $headers['Last-Modified'] ) && false === $headers['Last-Modified'] ) {
			unset( $headers['Last-Modified'] );

			// In PHP 5.3+, make sure we are not sending a Last-Modified header.
			if ( function_exists( 'header_remove' ) ) {
				@header_remove( 'Last-Modified' );
			} else {
				// In PHP 5.2, send an empty Last-Modified header, but only as a
				// last resort to override a header already sent. #WP23021
				foreach ( headers_list() as $header ) {
					if ( 0 === stripos( $header, 'Last-Modified' ) ) {
						$headers['Last-Modified'] = '';
						break;
					}
				}
			}
		}

		foreach ( (array) $headers as $name => $field_value )
			@header("{$name}: {$field_value}");

		if ( $exit_required )
			exit();

		/**
		 * Fires once the requested HTTP headers for caching, content type, etc. have been sent.
		 *
		 * @since WP-2.1.0
		 *
		 * @param WP $this Current ClassicPress environment instance (passed by reference).
		 */
		do_action_ref_array( 'send_headers', array( &$this ) );
	}