Source: http://codex.wordpress.org/Using_Custom_Fields
WordPress has the ability to allow post authors to assign custom fields to a post. This arbitrary extra information is known as meta-data. This meta-data can include bits of information such as:
- Mood: Happy
- Currently Reading: Cinderella
- Listening To: Rock Around the Clock
- Weather: Hot and humid
With some extra coding, it is possible to achieve more complex actions, such as using the metadata to store an expiration date for a post.
Meta-data is handled with key/value pairs. The key is the name of the meta-data element. The value is the information that will appear in the meta-data list on each individual post that the information is associated with.
Keys can be used more than once per post. For example, if you were reading two different books (perhaps a technical book at work and a fiction at home), you could create a "reading" key and use it twice on the same post, once for each book.
Here is an example of what this information might look like on your post:
Currently Reading: Calvin and Hobbes
Today’s Mood: Jolly and Happy
Usage
Based upon our example above, let’s put this into action. We’ll add two custom fields, one called "Currently Reading" and the other "Today’s Mood". The following instructions will demonstrate how to add this information to a post using Custom Fields.
- After you have written your post, scroll down to the area titled Custom Fields.
- To create a new Custom Field called "Currently Reading", enter the text "Currently Reading" (without the quotes) in the text entry field titled Key.
- The newly created Key should now be assigned a Value, which in our case is the name of the book currently being read, "Calvin and Hobbes". Type "Calvin and Hobbes" in the Value field, again without the quotes.
- Click Add Custom Field button to save this custom information for that post.
To add your "Today’s Mood", repeat the process and add "Today’s Mood" to the key and a description of your mood in the value text boxes and click SAVE to save this information with the post.
On your next post, you can add a new book and mood to your meta-data. In the Custom Fields section, the Key will now feature a pull down list with the previously entered Custom Fields. Choose "Currently Reading" and then enter the new book you are reading in the value. Click Add Custom Field and then repeat the process to add "Today’s Mood".
You only need to create a new "KEY" once, after which you can assign a value to that key for every post, if you so desire. You can also assign more than one Value to a key, for a post. This will come in handy for people who read more than one book at a time.
Displaying Custom Fields
With a Custom Field added to the post, it’s time to display your books and mood to the world. To display the Custom Fields for each post, use the the_meta() template tag. The tag must be put within The Loop in order to work. Many people add the_meta() template tag to the end of their post or in their Post Meta Data Section. Here is a basic example of using the tag:
<?php the_meta(); ?>
It might look like this in the source code:
<ul class='post-meta'>
<li><span class='post-meta-key'>Curently Reading:</span> Calvin and Hobbes</li>
<li><span class='post-meta-key'>Today's Mood:</span> Jolly and Happy</li>
</ul>
The template tag automatically puts the entire meta-data into a CSS style called post-meta. The key is in a span called post-meta-key so you can style it in your style sheet. All of this is showcased in an unordered list.
To customize the look of the post-meta list, change the characteristics in your style sheet. For instance, let’s add some style to our example from the top. The style sheet elements would look like this:
.post-meta {font-variant: small-caps; color: maroon; }
.post-meta-key {color: green; font-weight: bold; font-size: 110%; }
- Currently Reading: Calvin and Hobbes
- Today’s Mood: Jolly and Happy
There are also many WordPress Plugins in the Official WordPress Plugin Directory that add some nice features to the job of displaying meta tags. A search for Custom Field plugins at Google should help you find even more.
Advanced Techniques for Custom Fields
The following are more advanced techniques for getting and customizing meta-data and custom fields.
Getting Custom Fields
To fetch meta values use the get_post_meta() function:
get_post_meta($post_id, $key, $single);
- $post_id is the ID of the post you want the meta values for. Use $post->ID to get a post’s ID.
- $key is a string containing the name of the meta value you want.
- $single can either be true or false. If set to true then the function will return a single result, as a string. If false, or not set, then the function returns an array of the custom fields.
Implementation Details
The PostMeta information is stored in a new table, $wpdb->postmeta. This table has four fields:
- meta_id: A unique id for each entry
- post_id: The ID of the post for this metadata
- meta_key: The name of the ‘key’
- meta_value: The value associated with the key
The values from this table are pulled into a structured multi-dimensional array called $post_meta_cache, just after the $posts array is fetched in wp-blog-header.php. This variable will only contain values for the list of posts fetched for the current page build. The structure of the array will look something like this:
[
postid1 => [
[
key1 => [val1, val2, ...],
key2 => [val1, val2, ...],
...
],
postid2 => [ ... ],
...
]
So, if you wanted to fetch the "reading" values from post number 256, you use this PHP code:
// Fetch an array of values for what I'm reading:
$readinglist = $post_meta_cache[256]['reading'];
- Don’t forget that $readinglist will be an array, not a single value.
- As of WordPress 2.1, $post_meta_cache isn’t populated anymore. Get the meta values through the functions mentioned below
PostMeta Functions
Internal Functions
These functions are intended for use inside The Loop, and all return arrays.
- get_post_custom()
- Get all key/value data for the current post.
- get_post_custom_keys()
- Get a list of all key names for the current post.
- get_post_custom_values($key)
- Get the list of values for a particular key on the current post.
- get_post_meta($post_id, $key, $single = false)
- In WP 1.5 and beyond, this function returns the meta information without cache problems. The function requires the post id, the key, and if $single is set to TRUE, it returns only the first result (NOT as an array) for PHP use.
This will output the resulting meta value (notice the addition of "echo"):
<?php $key="mykey"; echo get_post_meta($post->ID, $key, true); ?>
Template Functions
At the time of this writing, there is only one template function.
- the_meta()
- Echoes an unordered list containing the current post’s meta-data with a class for the UL as post-meta and the LI as post-meta-key.
We expect that independent developers will come up with many interesting uses for post meta-data in the form of plugins. The the_meta() template function is just an extremely basic example.
At this time, you can only add and delete entries. The ability to modify existing entries will be implemented later.
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