Most Important WordPress Function

wp_function.php

1. Get Header
==============
  <?php get_header( $name ); ?>

=>Parameters
  # $name (Calls for header-name.php)

=> Different header for different pages.
<?php
if ( is_home() ) :
	get_header( 'home' );
elseif ( is_404() ) :
	get_header( '404' );
else :
	get_header();
endif;
?>

2. Get Footer
=============
<?php get_footer( $name ); ?>

3. Get Sidebar
==============
<?php get_sidebar( string $name = null ) ?>

=> Different sidebar for different pages.
<?php
if ( is_home() ) :
  get_sidebar( 'home' );
elseif ( is_404() ) :
  get_sidebar( '404' );
else :
  get_sidebar();
endif;
?>

4. Load Theme Textdomain
========================
  <?php load_theme_textdomain( $domain, $path ) ?>

  Example: <?php load_theme_textdomain( 'crazyland', get_template_directory() . '/languages' ); ?>

Here,
  ** Theme name is $domain name, 
  ** Directory Included in second parameter 

5. Add Theme Support (Theme Support)
=====================================
  <?php add_theme_support( string $feature ) ?>

#Source File: wp-includes/theme.php

=> Description  
  *** Must be called in the theme’s functions.php file to work. 
  *** If attached to a hook, it must be ‘after_setup_theme’. 
  *** The ‘init’ hook may be too late for some features.

#Version	Description

  4.7.0 =>	The starter-content feature was added
  4.5.0 =>	The customize-selective-refresh-widgets feature was added
  4.1.0 =>	The title-tag feature was added
  3.9.0 =>	The html5 feature now also accepts 'gallery' and 'caption'
  3.6.0 =>	The html5 feature was added
  2.9.0 =>	Introduced.

  => add_theme_support( 'post-thumbnails' );
  => add_theme_support( 'post-thumbnails', array( 'post' ) );          // Posts only
  => add_theme_support( 'post-thumbnails', array( 'page' ) );          // Pages only
  => add_theme_support( 'post-thumbnails', array( 'post', 'movie' ) ); // Posts and Movies
  => add_theme_support( 'custom-background' );

=> Note that you can add default arguments using:
$defaults = array(
    'default-image' => '',
    'default-preset' => 'default',
    'default-position-x' => 'left',
    'default-position-y' => 'top',
    'default-size' => 'auto',
    'default-repeat' => 'repeat',
    'default-attachment' => 'scroll',
    'default-color' => '',
    'wp-head-callback' => '_custom_background_cb',
    'admin-head-callback' => '',
    'admin-preview-callback' => '',
);
add_theme_support( 'custom-background', $defaults );

=> add_theme_support( 'custom-header' );

=> Note that you can add default arguments using:
$defaults = array(
    'default-image' => '',
    'random-default' => false,
    'width' => 0,
    'height' => 0,
    'flex-height' => false,
    'flex-width' => false,
    'default-text-color' => '',
    'header-text' => true,
    'uploads' => true,
    'wp-head-callback' => '',
    'admin-head-callback' => '',
    'admin-preview-callback' => '',
    'video' => false,
    'video-active-callback' => 'is_front_page',
);
add_theme_support( 'custom-header', $defaults );

=> add_theme_support( 'custom-logo' );

=> Note that you can add default arguments using:
add_theme_support( 'custom-logo', array(
    'height'      => 100,
    'width'       => 400,
    'flex-height' => true,
    'flex-width'  => true,
    'header-text' => array( 'site-title', 'site-description' ),
) );

=> add_theme_support( 'automatic-feed-links' );
=> add_theme_support( 'html5', array( 'comment-list', 'comment-form', 'search-form', 'gallery', 'caption' ) );
=> add_theme_support( 'title-tag' );
=> add_theme_support( 'customize-selective-refresh-widgets' );



# To check if there is a post thumbnail assigned to the post before displaying it, use:
<?php
if ( has_post_thumbnail() ) {
    the_post_thumbnail();
}
?>

6. Sidebar Widget:
==================
  <?php register_sidebar( $args ); ?>

  function theme_slug_widgets_init() {
    register_sidebar( array(
        'name' => __( 'Main Sidebar', 'theme-slug' ),
        'id' => 'sidebar-1',
        'description' => __( 'Widgets in this area will be shown on all posts and pages.', 'theme-slug' ),
        'before_widget' => '<li id="%1$s" class="widget %2$s">',
	'after_widget'  => '</li>',
	'before_title'  => '<h2 class="widgettitle">',
	'after_title'   => '</h2>',
    ) );
  }

  add_action( 'widgets_init', 'theme_slug_widgets_init' );

  => Builds the definition for a single sidebar and returns the ID. Call on "widgets_init" action.

7. WP Enqueue Style:
=====================
  wp_enqueue_style( $handle, $src = '', $deps = array(), $ver = false, $media = 'all' ) 
  => Ref: wp-includes/general-template.php: wp_enqueue_code_editor()

  wp_enqueue_script( string $handle, string $src = '', array $deps = array(), string|bool|null $ver = false, bool $in_footer = false )
  => e.g: wp_enqueue_script( 'script-name', get_template_directory_uri() . '/js/example.js', array(), '1.0.0', true );

8. Archive Pages
================

** Archive Pages Function:

<?php the_archive_title( string $before = '', string $after = '' ) ?>
=> Display the archive title based on the queried object.

<?php the_archive_description( string $before = '', string $after = '' ) ?>
=> Display category, tag, term, or author description.

9. Post Thumbnail:
==================
<?php has_post_thumbnail( int|WP_Post $post = null ) ?>
=> Check if post has an image attached.
=> Example:  

   <?php
      // Must be inside a loop.
    if ( has_post_thumbnail() ) {
        the_post_thumbnail();
    }
    else {
        echo '<img src="' . get_bloginfo( 'stylesheet_directory' ) 
            . '/images/thumbnail-default.jpg" />';
    }
    ?>

10. Permalink:
==============
<?php the_permalink(); ?> 
=> Displays the URL to the post, without creating a link:

<?php get_permalink( int|WP_Post $post, bool $leavename = false ) ?>
=> Retrieves the full permalink for the current post or post ID.

*** Used as Link With Title Tag:

<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a>

***  <?php the_title_attribute( $args ); ?>
=> Parameters: $before, $after, $echo, $post

Example: <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute( 'before=Permalink to: "&after="' ); ?>">
	    <?php the_title(); ?>
	 </a>