Source

File: wp-includes/class-wp.php

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

		if ( is_user_logged_in() ) {
			$headers = array_merge( $headers, wp_get_nocache_headers() );
		} elseif ( ! empty( $_GET['unapproved'] ) && ! empty( $_GET['moderation-hash'] ) ) {
			// Unmoderated comments are only visible for one minute via the moderation hash.
			$headers['Expires']       = gmdate( 'D, d M Y H:i:s', time() + MINUTE_IN_SECONDS );
			$headers['Cache-Control'] = 'max-age=60, must-revalidate';
		}
		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 ), true ) ) {
				$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 = gmdate( '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 2.8.0
		 *
		 * @param string[] $headers Associative array of headers to be sent.
		 * @param WP       $this    Current WordPress 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'] );

			if ( ! headers_sent() ) {
				header_remove( 'Last-Modified' );
			}
		}

		if ( ! headers_sent() ) {
			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 2.1.0
		 *
		 * @param WP $this Current WordPress environment instance (passed by reference).
		 */
		do_action_ref_array( 'send_headers', array( &$this ) );
	}