Wordpressheader. php documents

Wordpressheader. php file. When you are a beginner in php, you are still confused about the header file (location: wp-content/temes/twentyeleven/header. php) when reading wordpress code ). For more information, see in wordpress header. php.

When you are a beginner in php, you are still confused about the header file (location: wp-content/temes/twentyeleven/header. php) when reading wordpress code ).
I would like to ask you:
In the code of this file and include operations, how can external functions such as bloginfo () and wp_head () be called?
How can this development mode be implemented? I hope you will be grateful for your explanation or materials.

PHP code

  
   section and everything up till  * * @package WordPress * @subpackage Twenty_Eleven * @since Twenty Eleven 1.0 */?>
  
  
  
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  <?php    /*     * Print the <title> tag based on what is being viewed.     */    global $page, $paged;    wp_title( '|', true, 'right' );    // Add the blog name.    bloginfo( 'name' );    // Add the blog description for the home/front page.    $site_description = get_bloginfo( 'description', 'display' );    if ( $site_description && ( is_home() || is_front_page() ) )        echo " | $site_description";    // Add a page number if necessary:    if ( $paged >= 2 || $page >= 2 )        echo ' | ' . sprintf( __( 'Page %s', 'twentyeleven' ), max( $paged, $page ) );    ?>
  
  
  
  
       * 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();?>>    
              
   
                    " title="
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   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;                        }                        ?>                    " width="
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     'primary' ) ); ?>