WPSE 188373

gistfile1.php

<?php

function fb_modify_author_meta_boxes() {
	remove_meta_box('authordiv', 'post', 'normal');
	add_meta_box('fb_authordiv', __('Author'), 'fb_post_author_meta_box', 'post', 'normal', 'core');
}

add_action( 'admin_menu', 'fb_modify_author_meta_boxes' );

function fb_post_author_meta_box( $post ) {
	global $user_ID;
	
	?>
	
	<label class="screen-reader-text" for="post_author_override"><?php _e('Author'); ?></label>
	
<?php
	wp_dropdown_users( array(
    	'name' => 'post_author_override',
    	'selected' => empty( $post->ID ) ? $user_ID : $post->post_author,
    	'include_selected' => true
	));

	$show_author = !! get_post_meta( $post->ID, 'show_author', true ); 
?>

<input type="checkbox" name="show_author" <?php checked( $show_author ) /* WordPress helper function */ ?> style="margin-left:15px" /> Check the box to display the author's byline and author box.
<input type="hidden" name="do_show_author" value="1" />

<?php
}

/* Save checkbox status */
function save_details( $post_ID ) {
    if ( isset( $_POST['do_show_author'] ) ) {
        // Use isset - unchecked checkboxes won't send a value to $_POST, you'll get an undefined index error
        update_post_meta( $post_ID, 'show_author', isset( $_POST['show_author'] ) ? '1' : '0' );
    }
}

add_action( 'save_post', 'save_details' );