As we have seen in our previous post, WordPress Custom Fields Part I, custom fields provide an excellent way to add flexible content to your posts and pages. By assigning various types of content to different custom fields, you gain complete control over when, where, and how to display the associated information. For example, sub-headings may be displayed in the sidebar, footnotes may be consolidated into a single region, post images may be displayed before the post title, and so on. In this follow-up article, we will review the basics of custom fields and then jump into a few custom-field tips and tricks.

Quick review of custom fields

Custom fields may be added when you create or edit any page or post. Each custom field consists of two variables, the key and its associated value. An example key would be “current mood”, and an example value would be “happy”. Each custom field remains associated with its corresponding post or page, but may of course be called apart from the loop and displayed anywhere on the page.

There are several ways to retrieve and display custom field information on your pages. The first and easiest way uses the the_meta(); template tag, which always echoes an unordered list containing the attributes/values seen in this example:

By default, <?php the_meta(); ?> gives us this:

<ul class='post-meta'>
<li><span class='post-meta-key'>Key 1:</span> Value for Key 1</li>
<li><span class='post-meta-key'>Key 2:</span> Value for Key 2</li>
<li><span class='post-meta-key'>Key 3:</span> Value for Key 3</li>
</ul>

Although this method is useful for general purposes, anything involving greater degrees of customization will require something a little more flexible. Fortunately, the get_post_meta() template tag provides the flexibility needed for advanced configurations. There are many ways to use this template tag, so let’s break away from the basics and explore some advanced tips and tricks.

Display values of a specific key

To loop through and display the values of a specific key, place the following within the loop of your choice, and change the “mood” value to that of your desired key value:

<?php echo get_post_meta($post->ID, 'mood', true); ?>

Display multiple values of a specific key

Each custom-field key may include multiple values. For example, if you listen to multiple songs during a given post, you may want to list them all with a key of “songs”. Then to loop through and display the multiple values for the songs key, we place the following code into the loop of choice:

<?php $songs = get_post_meta($post->ID, 'songs', false); ?>
	<h3>This post inspired by:</h3>
	<ul>
		<?php foreach($songs as $song) {
			echo '<li>'.$song.'</li>';
			} ?>
	</ul>

Notice the trick here: by changing the third parameter to “false”, we tell the function to return an array of the values for the specified key. A very handy trick for displaying multiple key values.

Display content only if a custom field exists

For cases when not all posts contain some specific custom-field key, use the following code to prevent unwanted, empty or incomplete markup from destroying the validity of your page:

// display an image based on custom-field value, if it exists

<?php $image = get_post_meta($post->ID, 'url', true);
	
	if($image) : ?>

	<img src="<?php echo $image; ?>" alt="" />

	<?php endif; ?>

Conditional display of custom-field data

Continuing with the previous technique, here is a basic code template for displaying a list of key values only if they exist:

<?php if(get_post_meta($post->ID, 'books', true) || 
	 get_post_meta($post->ID, 'music', true) || 
	 get_post_meta($post->ID, 'sites', true)
	 ): ?>

	<ul>
		<?php if(get_post_meta($post->ID, 'books', true)): ?>
		<li><?php echo get_post_meta($post->ID, 'books', true); ?></li>
		<?php endif; ?>

		<?php if(get_post_meta($post->ID, 'music', true)): ?>
		<li><?php echo get_post_meta($post->ID, 'music', true); ?></li>
		<?php endif; ?>

		<?php if(get_post_meta($post->ID, 'sites', true)): ?>
		<li><?php echo get_post_meta($post->ID, 'sites', true); ?></li>
		<?php endif; ?>
	</ul>

<?php endif; ?>

More conditional content based on custom-field values

Here’s another neat trick whereby custom-field values are used to determine which type of content appears on a page. In this example, we are checking the value of of a custom-field key called “hobbies”. Depending on the value of the hobbies key, different links are output on the page. Check it out:

<?php $value = get_post_meta($post->ID, 'hobbies', true);

	if($value == 'gaming') {
		echo '<a href="http://domain.tld/gaming/">Gaming Stuff</a>';
	} elseif($value == 'sleeping') {
		echo '<a href="http://domain.tld/sleeping/">Nap Supplies</a>';
	} elseif($value == 'eating') {
		echo '<a href="http://domain.tld/eating/">Dieting Advice</a>';
	} else {
		echo '<a href="http://domain.tld/">Home Page</a>';
	}

?>

Simplification and externalization

To clean up our source code a little, we can relocate the get_post_meta() function to the theme’s functions.php file. The immediate benefit here is one less parameter to include in the template tag. To do this, first place the following code into your theme’s functions.php file:

<?php function get_custom_field_data($key, $echo = false) {
	global $post;
	$value = get_post_meta($post->ID, $key, true);
	if($echo == false) {
		return $value;
	} else { 
		echo $value;
	}
}
?>

..and then call the function by placing this code in the desired location within your page template:

<?php if(function_exists('get_custom_field_data')) {
	get_custom_field_data('key', true);
} ?>

The only thing you need to edit here is the value of the “key” parameter, which should be the same as the key for which you would like to display value data. The second parameter is currently set as “true” so that the key value is echoed to the browser. To save the key value as a variable for further processing, change this parameter to “false”.

Streamlining attribute values

Using the same principle as described in the previous example, we can create a function that will streamline the display of custom-field images while providing localized control over their associated (X)HTML attributes. Given the typical example of a custom-field value containing a URL to a specific image, we create function whereby the image URL is retrieved and displayed along with a set of attribute values passed from the function call. We place this function in our theme’s functions.php file:

function get_attribute_data($key, $alt, $title, $width, $height) {
	global $post;
	$value = get_post_meta($post->ID, $key, true);
	if($value) {
		echo '<img src="'.$value.'" alt="'.$alt.'" title="'.$title.'" width="'.$width.'" height="'.$height.'" />';
	} else {
		return;
	}
}

We then place the following function call into the desired location within our page template file:

<?php get_attribute_data('image', 'Alt text for the image', 'Title text for the image', 150, 150); ?>

Once in place, this function first checks for the value of a custom-field key named “image”. If such a value exists, it is echoed to the browser within the requisite image ( <img> ) markup, which is also populated with the attribute values specified in the function call. The usefulness of this technique may also be applied to other types of custom-field values, such as links, lists, and so on.

Additional internal custom-field functions

In addition to the get_post_meta() function, there are three additional PostMeta functions that return arrays when used inside of the loop:

• get_post_custom() — Returns an array of all key/value data for current post
• get_post_custom_keys() — Returns an array of all key data for current post
• get_post_custom_values($key) — Returns all values for a specific key for current post

Closing Thoughts

Hopefully at this point you have a clear understanding of how to implement custom fields. By generalizing the techniques described in this article and the previous tutorial, we may integrate virtually any type of content, associate it with any array of posts, and display the related content in segregated fashion according to the purposes of our design. Even better, using custom fields for particular types of content — featured images, footnotes, thumbnails, and other extra information — makes it easy to change the layout of your content on a sitewide basis.

References

• WordPress Codex: Template Tags/the meta
• WordPress Codex: Function Reference/get post meta