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    Creating Maintainable WordPress Meta Boxes: Verify and Sanitize

    4elements | web design Den Haag blog • Creating Maintainable WordPress Meta Boxes: Verify and Sanitize

    Throughout this series, we've been creating a plugin that's meant to provide authors with a way to collect, manage, and save ideas and references to content that they're creating within WordPress.

    While doing so, we're also looking at ways that we can organize our plugin so that the code and the file organization is clear and maintainable so that as the plugin continues development, we're able to easily add, remove, and maintain its features.

    Up to this point, we've put together the basic file organization of the plugin as well as the front-end, but we haven't actually implemented functionality for saving information to the database. And if you can't save information, then the plugin is of little benefit to anyone.

    In this post, we're going to hop back into the server-side code and begin implementing the functionality that will:

    1. Verify the user has the ability to save post meta data
    2. Sanitize the post meta data
    3. Save the post meta data
    4. Validate and retrieve the post meta data

    We've got our work cut out for us. In this article, we're going to be looking at the first two steps and then in the next post, we'll be looking at the final two steps.

    Verifying Permissions

    In order to verify that the user has the ability to publish to save post meta data, we need to implement a security check during the serialization process. In order to do this, we need to take advantage of a nonce value.

    A nonce is a "number used once" to protect URLs and forms from being misused.

    1. Add a Nonce

    In order to introduce one into our meta box, we can implement the functionality in the markup that's responsible for rendering the post template. To do this, load admin/views/authors-commentary-navigation.php and update the template so that it includes a call to wp_nonce_field:

    <div id="authors-commentary-navigation">
        <h2 class="nav-tab-wrapper current">
    		<a class="nav-tab nav-tab-active" href="javascript:;">Draft</a>
    		<a class="nav-tab" href="javascript:;">Resources</a>
    		<a class="nav-tab" href="javascript:;">Published</a>
    		// Include the partials for rendering the tabbed content
    		include_once( 'partials/drafts.php' );
    		include_once( 'partials/resources.php' );
    		include_once( 'partials/published.php' );
    		// Add a nonce field for security
    		wp_nonce_field( 'authors_commentary_save', 'authors_commentary_nonce' );

    In the code above, we've introduced a nonce that corresponds to the action of saving the author's commentary (which we've named authors_commentary_nonce) and associated it with a value that's identified by authors_commentary.

    We'll see where this comes into play momentarily. For now, if you load your browser, you won't see anything new display. That's because the nonce values are displayed in a hidden field. 

    For those who are curious, you can launch your favorite browser's development tools, inspect the meta box, and you should find something like the following in the markup:

    <input type="hidden" id="authors_commentary_nonce" name="authors_commentary_nonce" value="f3cd131d28">

    Of course, the value of your nonce will be different.

    2. Check the Nonce

    In order to make sure the user has permission to save the post, we want to check three things:

    1. that the user is saving information for a post post type
    2. that the post is not being automatically saved by WordPress
    3. that the user actually has permission to save

    We'll write two helper functions to achieve the first and third, and we'll use some built-in functions to check number two (which will actually be used in the second helper function).

    First, let's go ahead and setup the hook and the function that will be used to leverage the helper functions and save the meta data. In the constructor for Authors_Commentary_Meta_Box, add the following line of code:

    <?php add_action( 'save_post', array( $this, 'save_post' ) ); ?>

    Next, let's define the function. Note that I'm making calls to two functions in the following block of code. We'll be defining them momentarily:

     * Sanitizes and serializes the information associated with this post.
     * @since    0.5.0
     * @param    int    $post_id    The ID of the post that's currently being edited.
    public function save_post( $post_id ) {
        /* If we're not working with a 'post' post type or the user doesn't have permission to save,
    	 * then we exit the function.
    	if ( ! $this->is_valid_post_type() || ! $this->user_can_save( $post_id, 'authors_commentary_nonce', 'authors_commentary_save' ) ) {

    Given the code above, we're telling WordPress to fire our save_post function whenever its save_post action is called. Inside of the function, we're saying "If the post that's being saved is not a 'post' post type, or if the user does not have permission to save, then exit the function."

    Of course, we need to define the functions so that the logic works. First, we'll write the is_valid_post_type function as a private function of the current class. It will check the $_POST array to ensure that the type of post that's being saved is, in fact, a post.

     * Verifies that the post type that's being saved is actually a post (versus a page or another
     * custom post type.
     * @since       0.5.0
     * @access      private
     * @return      bool      Return if the current post type is a post; false, otherwise.
    private function is_valid_post_type() {
        return ! empty( $_POST['post_type'] ) && 'post' == $_POST['post_type'];

    Next, we'll add the user_can_save function. This is the function that will ensure that the post isn't being saved by WordPress, and that if a user is saving the function, then the nonce value associated with the post action is properly set.

     * Determines whether or not the current user has the ability to save meta data associated with this post.
     * @since       0.5.0
     * @access      private
     * @param		int		$post_id	  The ID of the post being save
     * @param       string  $nonce_action The name of the action associated with the nonce.
     * @param       string  $nonce_id     The ID of the nonce field.
     * @return		bool				  Whether or not the user has the ability to save this post.
    private function user_can_save( $post_id, $nonce_action, $nonce_id ) {
        $is_autosave = wp_is_post_autosave( $post_id );
        $is_revision = wp_is_post_revision( $post_id );
        $is_valid_nonce = ( isset( $_POST[ $nonce_action ] ) && wp_verify_nonce( $_POST[ $nonce_action ], $nonce_id ) );
        // Return true if the user is able to save; otherwise, false.
        return ! ( $is_autosave || $is_revision ) && $is_valid_nonce;

    Notice here that we're passing in the nonce_action and the nonce_id that we defined in the template in the first step. We're also using wp_verify_nonce in conjunction with said information.

    This is how we can verify that the post that's being saved is done so by a user that has the proper access and permissions.

    Sanitize the Data

    Assuming that the user is working with a standard post type and that the s/he has permission to save information, we need to sanitize the data.

    To do this, we need to do this following:

    1. Check to make sure that none of the information in the post meta data is empty
    2. Strip our anything that could be dangerous to write to the database

    After we do this, then we'll look at saving the information for each of the meta boxes. But first, let's work on sanitization. There are a couple of ways that we can go about implementing this. For the purposes of this post, we'll do it in the most straightforward way possible: We'll check for the existence of the information based on its key, then, if it exists, we'll sanitize it.

    For experienced programmers, you're likely going to notice some code smells with the code we're about to write. Later in this series, we'll be doing some refactoring to see how we can make the plugin more maintainable so it's all part of the intention of this particular post.

    With that said, hop back into the save_post function.

    1. Drafts

    Since the first tab that exists within the meta box is the Drafts tab, we'll start with it. Notice that it's a textarea, so the logic that exists for sanitizing that information should be as follows:

    • remove any HTML tags
    • escape the contents of the text area

    Recall that the textarea is named authors-commentary-drafts so that we can access it within the $_POST array. To achieve this, we'll use the following code:

    // If the 'Drafts' textarea has been populated, then we sanitize the information.
    if ( ! empty( $_POST['authors-commentary-drafts'] ) ) {
        // We'll remove all white space, HTML tags, and encode the information to be saved
    	$drafts = trim( $_POST['authors-commentary-drafts'] );
    	$drafts = esc_textarea( strip_tags( $drafts ) );
    	// More to come...

    Simply put, we're checking to see if the information in the $_POST array is empty. If not, then we'll sanitize the data.

    2. Resources

    This particular field is a little more form because it's dynamic. That is, the user can have anything from zero-to-many input fields all of which we'll need to manage. Remember that this particular tab is designed to primarily be for URLs so we need to make sure that we're safely sanitizing the information in that way.

    First, we need to make one small change to the createInputElement function that exists within the admin/assets/js/resources.js file. Specifically, we need to make sure that the name attribute is using an array so that we can properly access it and iterate through it when looking at $_POST data.

    Make sure that the lines of code responsible for creating the actual element looks like this:

    // Next, create the actual input element and then return it to the caller
    $inputElement =
        $( '<input />' )
    		.attr( 'type', 'text' )
    		.attr( 'name', 'authors-commentary-resources[' + iInputCount + ']' )
    		.attr( 'id', 'authors-commentary-resource-' + iInputCount )
    		.attr( 'value', '' );

    Notice that the key to what we've done lies in the line that updates the name. Specifically, we're placing the number of inputs an indexes of the array.

    Next, hop back into the save_post function and add the following code (which we'll discuss after the block):

    // If the 'Resources' inputs exist, iterate through them and sanitize them
    if ( ! empty( $_POST['authors-commentary-resources'] ) ) {
        $resources = $_POST['authors-commentary-resources'];
    	foreach ( $resources as $resource ) {
    		$resource = esc_url( strip_tags( $resource ) );
    		// More to come...

    Because we're working with an array of inputs, we need to first check that the array isn't empty. If it's not, then we need to iterate through it because we aren't sure how many inputs we're going to have to manage.

    Similar to the previous block, we're doing a basic level of sanitization and escaping. This is something that you can make as aggressive or as relaxed as you'd like. We'll be coming back to this conditional in the next post when it's time to save the data.

    3. Published

    This tab is similar to the previous tabs in that we're dealing with an indeterminate number of elements that we need to sanitize. This means that we're going to need to make a small update to the partial responsible for rendering this input.

    On the upside, we're only dealing with a checkbox which has a boolean value of being checked or not (or, specifically, 'on' or empty) so sanitizing the information is really easy.

    First, let's update the partial. Locate admin/views/partials/published.php. Next, find the line that defines the input checkbox and change it so that it looks like this:

    <label for="authors-commentary-comment-<?php echo $comment->comment_ID ?>">
        <input type="checkbox" name="authors-commentary-comments[<?php echo $comment->comment_ID ?>]" id="authors-commentary-comment-<?php echo $comment->comment_ID ?>" />
    	This comment has received a reply.

    Notice that we've changed the name attribute so that it uses a an array with an index as its value. Next, we'll hop back into the save_post function one more time in order to introduce validation on this particular element:

    // If there are any values saved in the 'Resources' input, save them
    if ( ! empty( $_POST['authors-commentary-comments'] ) ) {
        $comments = $_POST['authors-commentary-comments'];
    	foreach ( $comments as $comment ) {
    		$comment = strip_tags( stripslashes( $comment ) );
    		// More to come...

    Just as we've done with the previous pieces of data, we first check to see if the content exists. If so, then we sanitize it to prepare it for saving. If it doesn't then we don't do anything.

    On To Saving

    At this point, we're positioned to take on the last two points of the series:

    1. Saving and Retrieving
    2. Refactoring

    Starting in the next post, we'll revisit the code that we've written in this post to see how we can save it to the database and retrieve it from the database in order to display it on the front-end.

    Next, we'll move on to refactoring. After all, part of writing maintainable code is making sure that it's well-organized and easily changeable. Since the code that we work with on a day-to-day basis has already been written and could stand to be refactored, we're going to see how to do that by the end of the series.

    In the meantime, review the code above, check out the source from GitHub, and leave any questions and comments in the field below. 

    martijn broeders

    founder/ strategic creative bij 4elements web design & consultancy
    e-mail: .(JavaScript must be enabled to view this email address)
    telefoon: 06 38 55 28 54
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