By
Justin Tadlock
from: http://wp.smashingmagazine.com/2011/10/04/create-custom-post-meta-boxes-wordpress/
What seems like one of the most complicated bits of functionality in
WordPress is adding meta boxes to the post editing screen. This
complexity only grows as more and more tutorials are written on the
process with weird loops and arrays. Even meta box “frameworks” have
been developed. I’ll let you in on a little secret though: it’s not that
complicated.
Creating custom meta boxes is extremely simple, at least it is once
you’ve created your first one using the tools baked into WordPress’ core
code. In this tutorial, I’ll walk you through everything you need to
know about meta boxes:
- Creating meta boxes.
- Using meta boxes with any post type.
- Handling data validation.
- Saving custom meta data.
- Retrieving custom meta data on the front end.
Note: When I use the term “post” throughout this tutorial, I’m
referring to a post of any post type, not just the default blog post
type bundled with WordPress.
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What is a post meta box?
A post meta box is a draggable box shown on the post editing screen.
Its purpose is to allow the user to select or enter information in
addition to the main post content. This information should be related to
the post in some way.
Generally, two types of data is entered into meta boxes:
- Metadata (i.e. custom fields),
- Taxonomy terms.
Of course, there are other possible uses, but those two are the most
common. For the purposes of this tutorial, you’ll be learning how to
develop meta boxes that handle custom post metadata.
What is post metadata?
Post metadata is data that’s saved in the
wp_postmeta table in the database. Each entry is saved as four fields in this table:
meta_id: A unique ID for this specific metadata.post_id: The post ID this metadata is attached to.meta_key: A key used to identify the data (you’ll work with this often).meta_value: The value of the metadata.
In the following screenshot, you can see how this looks in the database.
When you get right down to it, metadata is just key/value pairs saved
for a specific post. This allows you to add all sorts of custom data to
your posts. It is especially useful when you’re developing custom post
types.
The only limit is your imagination.
Note: One thing to keep in mind is that a single meta key can have
multiple meta values. This isn’t a common use, but it can be extremely
powerful.
Working with post metadata
By now, you’re probably itching to build some custom meta boxes.
However, to understand how custom meta boxes are useful, you must
understand how to add, update, delete, and get post metadata.
I could write a book on the various ways to use metadata, but that’s
not the main purpose of this tutorial. You can use the following links
to learn how the post meta functions work in WordPress if you’re
unfamiliar with them.
The remainder of this tutorial assumes that you’re at least familiar with how these functions work.
The setup
Before building meta boxes, you must have some ideas about what type
of metadata you want to use. This tutorial will focus on building a meta
box that saves a custom post CSS class, which can be used to style
posts.
I’ll start you off by teaching you to develop custom code that does a few extremely simple things:
- Adds an input box for you to add a custom post class (the meta box).
- Saves the post class for the
smashing_post_class meta key.
- Filters the
post_class hook to add your custom post class.
You can do much more complex things with meta boxes, but you need to learn the basics first.
All of the PHP code in the following sections goes into either your custom plugin file or your theme’s
functions.php file.
Building a custom post meta box
Now that you know what you’re building, it’s time to start diving
into some code. The first two code snippets in this section of the
tutorial are mostly about setting everything up for the meta box
functionality.
Since you only want your post meta box to appear on the post editor screen in the admin, you’ll use the
load-post.php and
load-post-new.php hooks to initialize your meta box code.
2 | add_action( 'load-post.php', 'smashing_post_meta_boxes_setup' ); |
3 | add_action( 'load-post-new.php', 'smashing_post_meta_boxes_setup' ); |
Most WordPress developers should be familiar with how hooks work, so
this should not be anything new to you. The above code tells WordPress
that you want to fire the
smashing_post_meta_boxes_setup function on the post editor screen. The next step is to create this function.
The following code snippet will add your meta box creation function to the
add_meta_boxes hook. WordPress provides this hook to
add meta boxes.
2 | function smashing_post_meta_boxes_setup() { |
5 | add_action( 'add_meta_boxes', 'smashing_add_post_meta_boxes' ); |
Now, you can get into the fun stuff.
In the above code snippet, you added the
smashing_add_post_meta_boxes() function to the
add_meta_boxes hook. This function’s purpose should be to add post meta boxes.
In the next example, you’ll create a single meta box using the
add_meta_box() WordPress function. However, you can add as many meta boxes as you like at this point when developing your own projects.
Before proceeding, let’s look at the
add_meta_box() function:
1 | add_meta_box( $id, $title, $callback, $page, $context = 'advanced', $priority = 'default', $callback_args = null ); |
$id: This is a unique ID assigned to your meta box. It should have a unique prefix and be valid HTML.$title: The title of the meta box. Remember to internationalize this for translators.$callback: The callback function that displays the output of your meta box.$page: The admin page to display the meta box on. In our case, this would be the name of the post type (post, page, or a custom post type).$context: Where on the page the meta box should be shown. The available options are normal, advanced, and side.$priority: How high/low the meta box should be prioritized. The available options are default, core, high, and low.$callback_args: An array of custom arguments you can pass to your $callback function as the second parameter.
The following code will add the post class meta box to the post editor screen.
02 | function smashing_add_post_meta_boxes() { |
05 | 'smashing-post-class', |
06 | esc_html__( 'Post Class', 'example' ), |
07 | 'smashing_post_class_meta_box', |
You still need to display the meta box’s HTML though. That’s where the
smashing_post_class_meta_box() function comes in (
$callback parameter from above).
02 | function smashing_post_class_meta_box( $object, $box ) { ?> |
04 | <?php wp_nonce_field( basename( __FILE__ ), 'smashing_post_class_nonce' ); ?> |
07 | <label for="smashing-post-class"><?php _e( "Add a custom CSS class, which will be applied to WordPress' post class.", 'example' ); ?></label> |
09 | <input class="widefat" type="text" name="smashing-post-class" id="smashing-post-class" value="<?php echo esc_attr( get_post_meta( $object->ID, 'smashing_post_class', true ) ); ?>" size="30" /> |
What the above function does is display the HTML output for your meta box. It displays a hidden nonce input (you can
read more about nonces
on the WordPress Codex). It then displays an input element for adding a
custom post class as well as output the custom class if one has been
input.
At this point, you should have a nice-looking meta box on your post
editing screen. It should look like the following screenshot.
The meta box doesn’t actually do anything yet though. For example, it
won’t save your custom post class. That’s what the next section of this
tutorial is about.
Saving the meta box data
Now that you’ve learned how to create a meta box, it’s time to learn how to save post metadata.
Remember that
smashing_post_meta_boxes_setup() function you created earlier? You need to modify that a bit. You’ll want to add the following code to it.
2 | add_action( 'save_post', 'smashing_save_post_class_meta', 10, 2 ); |
So, that function will actually look like this:
2 | function smashing_post_meta_boxes_setup() { |
5 | add_action( 'add_meta_boxes', 'smashing_add_post_meta_boxes' ); |
8 | add_action( 'save_post', 'smashing_save_post_class_meta', 10, 2 ); |
The new code you’re adding tells WordPress that you want to run a custom function on the
save_post hook. This function will save, update, or delete your custom post meta.
When saving post meta, your function needs to run through a number of processes:
- Verify the nonce set in the meta box function.
- Check that the current user has permission to edit the post.
- Grab the posted input value from
$_POST.
- Decide whether the meta should be added, updated, or deleted based on the posted value and the old value.
I’ve left the following function somewhat generic so that you’ll have
a little flexibility when developing your own meta boxes. It is the
final snippet of code that you’ll need to save the metadata for your
custom post class meta box.
02 | function smashing_save_post_class_meta( $post_id, $post ) { |
05 | if ( !isset( $_POST['smashing_post_class_nonce'] ) || !wp_verify_nonce( $_POST['smashing_post_class_nonce'], basename( __FILE__ ) ) ) |
09 | $post_type = get_post_type_object( $post->post_type ); |
12 | if ( !current_user_can( $post_type->cap->edit_post, $post_id ) ) |
16 | $new_meta_value = ( isset( $_POST['smashing-post-class'] ) ? sanitize_html_class( $_POST['smashing-post-class'] ) : '' ); |
19 | $meta_key = 'smashing_post_class'; |
22 | $meta_value = get_post_meta( $post_id, $meta_key, true ); |
25 | if ( $new_meta_value && '' == $meta_value ) |
26 | add_post_meta( $post_id, $meta_key, $new_meta_value, true ); |
29 | elseif ( $new_meta_value && $new_meta_value != $meta_value ) |
30 | update_post_meta( $post_id, $meta_key, $new_meta_value ); |
33 | elseif ( '' == $new_meta_value && $meta_value ) |
34 | delete_post_meta( $post_id, $meta_key, $meta_value ); |
At this point, you can save, update, or delete the data in the “Post Class” meta box you created from the post editor screen.
Using the metadata from meta boxes
So you have a custom post meta box that works, but you still need to
do something with the metadata that it saves. That’s the point of
creating meta boxes. What to do with your metadata will change from
project to project, so this is not something I can answer for you.
However, you will learn how to use the metadata from the meta box you’ve
created.
Since you’ve been building a meta box that allows a user to input a custom post class, you’ll need to filter WordPress’
post_class hook so that the custom class appears alongside the other post classes.
Remember that
get_post_meta() function from much earlier in the tutorial? You’ll need that too.
The following code adds the custom post class (if one is given) from your custom meta box.
02 | add_filter( 'post_class', 'smashing_post_class' ); |
04 | function smashing_post_class( $classes ) { |
07 | $post_id = get_the_ID(); |
10 | if ( !empty( $post_id ) ) { |
13 | $post_class = get_post_meta( $post_id, 'smashing_post_class', true ); |
16 | if ( !empty( $post_class ) ) |
17 | $classes[] = sanitize_html_class( $post_class ); |
If you look at the source code of the page where this post is shown
on the front end of the site, you’ll see something like the following
screenshot.
Pretty cool, right? You can use this custom class to style posts however you want in your theme’s stylesheet.
Security
One thing you should keep in mind when saving data is security.
Security is a lengthy topic and is outside the scope of this article.
However, I thought it best to at least remind you to keep security in
mind.
You’ve already been given a link explaining nonces earlier in this
tutorial. The other resource I want to provide you with is the WordPress
Codex guide on
data validation.
This documentation will be your best friend when learning how to save
post metadata and will provide you with the tools you’ll need for
keeping your plugins/themes secure.
Bonus points to anyone who can name all of the security measures used throughout this tutorial.
Create a custom meta box
Once you’ve copied, pasted, and tested the bits of pieces of code
from this tutorial, I encourage you to try out something even more
complex. If you really want to see how powerful meta boxes and post
metadata can be, try doing something with a single meta key and multiple
meta values for that key (it’s challenging).
I hope you’ve enjoyed the tutorial. Feel free to post questions about creating meta boxes in the comments.
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