It is pretty common knowledge that custom meta fields can be added to posts, pages and custom post types, but did you know that you can add custom meta fields to post tags, categories, and custom taxonomies as well? It’s actually pretty simple, though relatively undocumented. So in this quick tutorial I’m going to walk you through the process of added some custom meta fields to taxonomy. These meta fields can be used for a variety of purposes, but one of my favorites is the Taxonomy Images plugin by Michael Fields.

With WordPress 4.4, there will be a native “terms metadata” table in WordPress, so this is no longer a necessary or valid method of adding customer metadata to terms.

See here for more information: https://make.wordpress.org/core/2015/09/04/taxonomy-term-metadata-proposal/

Adding custom meta fields to a taxonomy requires three separate functions: one to add the field to the Add New page; one to add the field to the Edit Term page; and one to save the values of the custom field from both pages. This first function will add a custom field to the Add New page for the default Category taxonomy.

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<?php
// Add term page
function pippin_taxonomy_add_new_meta_field() {
	// this will add the custom meta field to the add new term page
	?>
	<div class="form-field">
		<label for="term_meta[custom_term_meta]"><?php _e( 'Example meta field', 'pippin' ); ?></label>
		<input type="text" name="term_meta[custom_term_meta]" id="term_meta[custom_term_meta]" value="">
		<p class="description"><?php _e( 'Enter a value for this field','pippin' ); ?></p>
	</div>
<?php
}
add_action( 'category_add_form_fields', 'pippin_taxonomy_add_new_meta_field', 10, 2 );

There is nothing special about this function. It’s just an HTML form that uses the same layout as the default WordPress add new term page fields. One thing to note, however, is that I have setup the name of the input to be an array. This is important for our save function. If the field name was not an array, then we’d have to save each field individually in the save function, but as an array, we can simply loop through the fields and save each one automatically.

Also take note of the add_action():

add_action( 'category_add_form_fields', 'pippin_taxonomy_add_new_meta_field', 10, 2 );

The first parameter is what determines the taxonomy that this field gets added to. It uses this format: {$taxonomy_name}_add_form_fields. So if you wanted to add the field to your “genres” taxonomy, you would use:

add_action( 'genres_add_form_fields', 'pippin_taxonomy_add_new_meta_field', 10, 2 );

You will now have a field that looks like this on the Add New term page:

The second function we need is the one that adds the HTML to our taxonomy Edit Term page. It looks very similar, though the HTML layout is a little different, and we also have to do a check to see if the meta field has any data saved already, so that we can populate the field on load.

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<?php
// Edit term page
function pippin_taxonomy_edit_meta_field($term) {
 
	// put the term ID into a variable
	$t_id = $term->term_id;
 
	// retrieve the existing value(s) for this meta field. This returns an array
	$term_meta = get_option( "taxonomy_$t_id" ); ?>
	<tr class="form-field">
	<th scope="row" valign="top"><label for="term_meta[custom_term_meta]"><?php _e( 'Example meta field', 'pippin' ); ?></label></th>
		<td>
			<input type="text" name="term_meta[custom_term_meta]" id="term_meta[custom_term_meta]" value="<?php echo esc_attr( $term_meta['custom_term_meta'] ) ? esc_attr( $term_meta['custom_term_meta'] ) : ''; ?>">
			<p class="description"><?php _e( 'Enter a value for this field','pippin' ); ?></p>
		</td>
	</tr>
<?php
}
add_action( 'category_edit_form_fields', 'pippin_taxonomy_edit_meta_field', 10, 2 );

First, we have passed the $term parameter to our function. This variable will contain the database object for our term. The only information we need for our function is the term ID, so we put that into a variable to make things easy. We then use the get_option() function to retrieve the value of out term meta. Note, all term meta is stored in an array in the option named “taxonomy_{$term_id}”. So to get the value of our specific meta field, we’d use $term_meta[‘custom_term_meta’];

The results of this function will look like this:

Next is our save function. This will take the value entered in the field and store it in the taxonomy option.

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// Save extra taxonomy fields callback function.
function save_taxonomy_custom_meta( $term_id ) {
	if ( isset( $_POST['term_meta'] ) ) {
		$t_id = $term_id;
		$term_meta = get_option( "taxonomy_$t_id" );
		$cat_keys = array_keys( $_POST['term_meta'] );
		foreach ( $cat_keys as $key ) {
			if ( isset ( $_POST['term_meta'][$key] ) ) {
				$term_meta[$key] = $_POST['term_meta'][$key];
			}
		}
		// Save the option array.
		update_option( "taxonomy_$t_id", $term_meta );
	}
}  
add_action( 'edited_category', 'save_taxonomy_custom_meta', 10, 2 );  
add_action( 'create_category', 'save_taxonomy_custom_meta', 10, 2 );

The function takes the ID of our term as a parameter. A quick check if performed to make sure that data is being sent from our custom field, and if it is, the current value of the field is retrieved from the database. Next, each of our custom fields (remember, we can setup more than one in a single array) is looped through. In the loop, each of the field values is inserted into the $term_meta array. Once the loop is complete, the taxonomy option is updated, storing our field values in the database.

Just like with the first two functions, the save function is tied to our particular taxonomy with an add_action() hook, and, again with the genres example, if you attached your fields to a taxonomy called “genres”, then you would use these hooks:

add_action( 'edited_genres', 'save_taxonomy_custom_meta', 10, 2 );  
add_action( 'create_genres', 'save_taxonomy_custom_meta', 10, 2 );

That’s it! The process is really pretty simple.

If you’d like to see a real world example of what you can do with these kind of meta fields (I already mentioned a plugin at the top), check out the Fotos theme by my buddy AJ Clarke. He has password protected photo galleries. The password is set via a taxonomy custom field for the gallery categories. I had the pleasure of building the system for him, so I can tell you first hand how well it works. I’d highly encourage you check it out.