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<?php

class Custom_Meta_Box {

    public function __construct() {

        if ( is_admin() ) {

            add_action( 'load-post.php', array( $this, 'init_metabox' ) );
            add_action( 'load-post-new.php', array( $this, 'init_metabox' ) );
        }
    }

    public function init_metabox() {

        add_action( 'add_meta_boxes', array( $this, 'add_metabox' ) );
        add_action( 'save_post', array( $this, 'save_metabox' ), 10, 2 );
    }

    public function add_metabox() {

        add_meta_box(
            'my-meta-box',
            __( 'My Meta Box', 'text_domain' ),
            array( $this, 'render_metabox' ),
            'post',
            'advanced',
            'default'
        );
    }

    public function render_metabox( $post ) {

        //* Add nonce for security and authentication...
        wp_nonce_field( 'custom_nonce_action', 'custom_nonce' );
    }

    public function save_metabox( $post_id, $post ) {

        //* Add nonce for security and authentication...
        $nonce_name   = isset( $_POST['custom_nonce'] ) ? $_POST['custom_nonce'] : '';
        $nonce_action = 'custom_nonce_action';

        //* Check if nonce is set...
        if ( ! isset( $nonce_name ) )
            return;

        //* Check if nonce is valid...
        if ( ! wp_verify_nonce( $nonce_name, $nonce_action ) )
            return;

        //* Check if user has permissions to save data...
        if ( ! current_user_can( 'edit_post', $post_id ) )
            return;

        //* Check if not an autosave...
        if ( wp_is_post_autosave( $post_id ) )
            return;

        //* Check if not a revision...
        if ( wp_is_post_revision( $post_id ) )
            return;
    }
}

new Custom_Meta_Box;