Add a 'minor edit' checkbox to Wordpress posts, pages and custom posts to save without updating the modified date

minor_edit.php

<?php

/*
 * Add a 'minor edit' checkbox to Wordpress posts, pages and custom posts
 * to save without updating the 'modified' date of the post.
 * Saved as postmeta for each post
 */
 
/* add checkbox to Publish box
--------------------------------------------------------------------------*/

add_action( 'post_submitbox_misc_actions', 'minor_edit_option' );

function minor_edit_option(){
	global $post;
  
  $val = get_post_meta( $post -> ID, 'minor_edit', true );
  
  wp_nonce_field( basename( __FILE__ ), 'minor_edit_nonce' );
  
  echo '<div class="misc-pub-section">';
  echo 	'<p>';
  echo 		'<input type="hidden" name="minor_edit" value="n" />'; 
  echo 		'<input type="checkbox" id="minor_edit" name="minor_edit" value="y" ' . checked( $val, 'y', false ) . ' /> ';
  echo 		'<label for="minor_edit"><strong>Minor Edit</strong></label>';
  echo 	'<p>';
  echo 	'<p>';
  echo 		'<small>Check to leave the \'modified\' date unaltered</small>';
  echo 	'<p>';
  echo '</div>';
}


/* On save, update minor edit option
--------------------------------------------------------------------------*/

add_action( 'save_post', 'minor_edit_save_postmeta', 10, 3 );

function minor_edit_save_postmeta( $post_id, $post_data, $action ){
	
	if ( !isset( $_POST['minor_edit_nonce'] ) || !wp_verify_nonce( $_POST['minor_edit_nonce'], basename( __FILE__ ) ) ):
		return;
	endif;
		
	$val = isset( $_POST['minor_edit'] ) ? $_POST['minor_edit'] : 'n';
	update_post_meta( $post_id, 'minor_edit', $val );

}


/* Keep previous modified date for post
--------------------------------------------------------------------------*/

add_action( 'post_updated', 'minor_edit', 10, 3 );

function minor_edit( $post_id, $post_after, $post_before ){
	
	global $wpdb;
	
	$minor_edit = get_post_meta( $post_id, 'minor_edit', true );
	
	if( $minor_edit == 'y' ):
		$wpdb -> update( 
			$wpdb -> prefix . 'posts', 
			array( 
				'post_modified' => $post_before -> post_modified,
				'post_modified_gmt' => $post_before -> post_modified_gmt
			), 
			array( 'ID' => $post_id ), 
			array( 
				'%s',
				'%s'
			), 
			array( '%d' ) 
		);
	endif;
	
}
?>