wordpress header.php文件的疑问。

在下php新手，在看wordpress代码时，对header文件始终不解（位置:wp-content/temes/twentyeleven/header.php）。
特来请教各位：
该文件代码中并include操作，其中如bloginfo(),wp_head()等外部函数如何可以调用？
此种开发模式如何实现？望各位能为在下讲解下，或提供资料，在下不甚感激。

PHP code

<?php /**
 * The Header for our theme.
 *
 * Displays all of the  section and everything up till 
 *
 * @package WordPress
 * @subpackage Twenty_Eleven
 * @since Twenty Eleven 1.0
 */
?>




>








<?php /* We add some JavaScript to pages with the comment form
     * to support sites with threaded comments (when in use).
     */
    if ( is_singular() && get_option( 'thread_comments' ) )
        wp_enqueue_script( 'comment-reply' );

    /* Always have wp_head() just before the closing 
     * tag of your theme, or you will break many plugins, which
     * generally use this hook to add elements to  such
     * as styles, scripts, and meta tags.
     */
    wp_head();
?>


>

    
<?php bloginfo( 'name' ); ?>
                
<?php bloginfo( 'description' ); ?>
            
<?php // Check to see if the header image has been removed
                $header_image = get_header_image();
                if ( $header_image ) :
                    // Compatibility with versions of WordPress prior to 3.4.
                    if ( function_exists( 'get_custom_header' ) ) {
                        // We need to figure out what the minimum width should be for our featured image.
                        // This result would be the suggested width if the theme were to implement flexible widths.
                        $header_image_width = get_theme_support( 'custom-header', 'width' );
                    } else {
                        $header_image_width = HEADER_IMAGE_WIDTH;
                    }
                    ?>
                <?php // The header image
                    // Check if this is a post or page, if it has a thumbnail, and if it's a big one
                    if ( is_singular() && has_post_thumbnail( $post->ID ) &&
                            ( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), array( $header_image_width, $header_image_width ) ) ) &&
                            $image[1] >= $header_image_width ) :
                        // Houston, we have a new header image!
                        echo get_the_post_thumbnail( $post->ID, 'post-thumbnail' );
                    else :
                        // Compatibility with versions of WordPress prior to 3.4.
                        if ( function_exists( 'get_custom_header' ) ) {
                            $header_image_width  = get_custom_header()->width;
                            $header_image_height = get_custom_header()->height;
                        } else {
                            $header_image_width  = HEADER_IMAGE_WIDTH;
                            $header_image_height = HEADER_IMAGE_HEIGHT;
                        }
                        ?>
                    " height="<?php echo $header_image_height; ?>" alt="" />
                <?php endif; // end check for featured image or standard header ?>
            <?php endif; // end check for removed header image ?><?php // Has the text been hidden?
                if ( 'blank' == get_header_textcolor() ) :
            ?>
                <?php get_search_form(); ?>

            <?php else :
            ?><?php get_search_form(); ?><?php endif; ?>