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Description

Adds a custom field (also called meta-data) to a specified post which could be of any post type. A custom field is effectively a key–value pair.

Note that if the given key already exists among custom fields of the specified post, another custom field with the same key is added unless the $unique argument is set to true, in which case, no changes are made. If you want to update the value of an existing key, use the update_post_meta() function instead.

Usage

 <?php add_post_meta($post_id, $meta_key, $meta_value, $unique); ?> 

Parameters

$post_id

(integer) (required) The ID of the post to which a custom field should be added.

Default: None

$meta_key

(string) (required) The key of the custom field which should be added.

Default: None

$meta_value

(mixed) (required) The value of the custom field which should be added. If an array is given, it will be serialized into a string.

Default: None

$unique

(boolean) (optional) Whether or not you want the key to stay unique. When set to true, the custom field will not be added if the given key already exists among custom fields of the specified post.

Default: false

Return

(boolean|integer) 

On success, returns the ID of the inserted row, which validates to true. If the $unique argument was set to true and a custom field with the given key already exists, false is returned.

Examples

Default Usage

<?php add_post_meta( 68, 'my_key', 47 ); ?>

Adding or Updating a Unique Custom Field

Add a new custom field if the key does not already exist, or updates the value of the custom field with that key otherwise.

<?php
if ( ! add_post_meta( 7, 'fruit', 'banana', true ) ) { 
   update_post_meta( 7, 'fruit', 'banana' );
}

Other Examples

Adds a new custom field only if a custom field with the given key does not already exist:

<?php add_post_meta( 68, 'my_key', '47', true ); ?>

Adds several custom fields with different values but with the same key 'my_key':

<?php add_post_meta( 68, 'my_key', '47' ); ?>
<?php add_post_meta( 68, 'my_key', '682' ); ?>
<?php add_post_meta( 68, 'my_key', 'The quick, brown fox jumped over the lazy dog.' ); ?>
...

For a more detailed example, see the post_meta Functions Examples page.

Hidden Custom Fields

If you are a plugin or theme developer and you are planning to use custom fields to store parameters related to your plugin or template, it is interesting to note that WordPress will not show custom fields which have keys starting with an "_" (underscore) in the custom fields list on the post edit screen or when using the the_meta() template function. This can be for example used to show these custom fields in an unusual way by using the add_meta_box() function.

The following example:

<?php add_post_meta( 68, '_color', 'red', true ); ?>

will add a unique custom field with the key name '_color' and the value 'red' but this custom field will not display in the post edit screen.

Hidden Arrays

In addition, if the $meta_value argument is an array, it will not be displayed on the page edit screen, even if you don't prefix the key name with an underscore.

Protected Custom Fields

An alternative to the use of an "_" (underscore) is to use the filter is_protected_meta.

Notes

• Uses add_metadata().

Character Escaping

Because meta values are passed through the stripslashes() function, you need to be careful about content escaped with \ characters. You can read more about the behavior, and a workaround example, in the update_post_meta() documentation.

Source Code

add_post_meta() is located in wp-includes/post.php

Change Log

Since 1.5.0

Related

Custom Fields: the_meta(), get_post_meta(), add_post_meta(), update_post_meta(), delete_post_meta(), get_post_custom(), get_post_custom_values(), get_post_custom_keys() (See Also: post_meta Function Examples)