How to create or register custom taxonomy like categories or tag for custom post type or default post type wordpress

For create a custom taxonomy for your custom post type just copy and paste below code into your theme's functions.php

add_action( 'init', 'rj_create_guide_taxonomy', 0 );


function rj_create_guide_taxonomy() {

$labels = array(
'name'              => _x( 'Categories', 'taxonomy general name' ),
'singular_name'     => _x( 'Category', 'taxonomy singular name' ),
'search_items'      => __( 'Search Categories' ),
'all_items'         => __( 'All Categories' ),
'parent_item'       => __( 'Parent Category' ),
'parent_item_colon' => __( 'Parent Category:' ),
'edit_item'         => __( 'Edit Category' ),
'update_item'       => __( 'Update Category' ),
'add_new_item'      => __( 'Add New Category' ),
'new_item_name'     => __( 'New Category Name' ),
'menu_name'         => __( 'Categories' ),
);

$args = array(
'hierarchical'      => true,
'labels'            => $labels,
'show_ui'           => true,
'show_admin_column' => true,
'query_var'         => true,
'rewrite'           => array( 'slug' => 'guide_cat' ),
);
// below will be create a taxonomy like categories (hierarchical)
register_taxonomy( 'guide_cat', array( 'guide' ), $args );


$labels = array(
'name'                       => _x( 'Tags', 'taxonomy general name' ),
'singular_name'              => _x( 'Tag', 'taxonomy singular name' ),
'search_items'               => __( 'Search Tags' ),
'popular_items'              => __( 'Popular Tags' ),
'all_items'                  => __( 'All Tags' ),
'parent_item'                => null,
'parent_item_colon'          => null,
'edit_item'                  => __( 'Edit Tag' ),
'update_item'                => __( 'Update Tag' ),
'add_new_item'               => __( 'Add New Tag' ),
'new_item_name'              => __( 'New Tag Name' ),
'separate_items_with_commas' => __( 'Separate tags with commas' ),
'add_or_remove_items'        => __( 'Add or remove tags' ),
'choose_from_most_used'      => __( 'Choose from the most used tags' ),
'not_found'                  => __( 'No tags found.' ),
'menu_name'                  => __( 'Tags' ),
);

$args = array(
'hierarchical'          => false,
'labels'                => $labels,
'show_ui'               => true,
'show_admin_column'     => true,
'update_count_callback' => '_update_post_term_count',
'query_var'             => true,
'rewrite'               => array( 'slug' => 'guide_tag' ),
);
// below will be create a taxonomy like tags(non hierarchical)
register_taxonomy( 'guide_tag', 'guide', $args );
}