PHP: Register Custom Post Type

gs-change-registered-cpt-init.php

<?php

add_action( 'init', 'gs_books_label_rename', 999 );
/**
 * Modify registered post type menu label
 * 
 */
function gs_books_label_rename() {
    global $wp_post_types;
    $wp_post_types['gs_books']->labels->menu_name = __( 'Books', 'gs_books' );
} 

gs-change-registered-cpt.php

<?php

add_action( 'registered_post_type', 'gs_books_label_rename', 10, 2 );
/**
 * Modify registered post type menu label
 * 
 * @param string $post_type Registered post type name. 
 * @param array $args Array of post type parameters.
 */
function gs_books_label_rename( $post_type, $args ) {
    if ( 'gs_books' === $post_type ) {
        global $wp_post_types;
        $args->labels->menu_name = __( 'Books', 'gs_books' );
        $wp_post_types[ $post_type ] = $args;
    }
} 

gs-register-custom-post-type.php

<?php

add_action( 'init', 'gs_register_books_cpt' );
/**
 * Register Books Custom Post Type 
 */
function gs_register_books_cpt() {
    // change 'gs_books' to whatever your text_domain is.
    
    /** Setup labels */
    $labels = array(
        'name'               => x_( 'Books', 'gs_books' ),
        'singular_name'      => x_( 'Book', 'gs_books' ),
        'add_new'            => x_( 'Add New', 'gs_books' ),
        'all_items'          => x_( 'All Books', 'gs_books' ),
        'add_new_item'       => x_( 'Add New Book', 'gs_books' ),
        'edit_item'          => x_( 'Edit Book', 'gs_books' ),
        'new_item'           => x_( 'New Book', 'gs_books' ),
        'view_item'          => x_( 'View Book', 'gs_books' ),
        'search_items'       => x_( 'Search Books', 'gs_books' ),
        'not_found'          => x_( 'No Books found', 'gs_books' ),
        'not_found_in_trash' => x_( 'No Books found in trash', 'gs_books' ),
        'parent_item_colon'  => x_( 'Parent Book:', 'gs_books' ),
        'menu_name'          => x_( 'Amazon Books', 'gs_books' )
    );
    
    /** Setup args */
    $args = array(
        'labels'              => $labels,
        'description'         => x_( 'Amazon Books post type', 'gs_books' ),
        'public'              => true,
        'menu_position'       => 20,
        'supports'            => array( 'title', 'editor', 'excerpt', 'page-attributes', ),
        'has_archive'         => 'books',
        'rewrite'             => array( 'slug' => 'book', ),
    );
    
    /** Register Custom Post Type */
    register_post_type( 'gs_books', $args );

}