add_post_meta( int $post_id, string $meta_key, mixed $meta_value, bool $unique = false ): int|false

Adds a meta field to the given post.

Description

Post meta data is called “Custom Fields” on the Administration Screen.

For historical reasons both the meta key and the meta value are expected to be “slashed” (slashes escaped) on input.

Parameters

$post_idintrequired

Post ID.

$meta_keystringrequired

Metadata name.

$meta_valuemixedrequired

Metadata value. Arrays and objects are stored as serialized data and will be returned as the same type when retrieved. Other data types will be stored as strings in the database:

• false is stored and retrieved as an empty string ('')
• true is stored and retrieved as '1'
• numbers (both integer and float) are stored and retrieved as strings Must be serializable if non-scalar.

$uniquebooloptional

Whether the same key should not be added.

Default:false

Return

More Information

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

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

function add_post_meta( $post_id, $meta_key, $meta_value, $unique = false ) {
	// Make sure meta is added to the post, not a revision.
	$the_post = wp_is_post_revision( $post_id );
	if ( $the_post ) {
		$post_id = $the_post;
	}

	return add_metadata( 'post', $post_id, $meta_key, $meta_value, $unique );
}

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Related

Changelog

User Contributed Notes

1. Skip to note 6 content
   Codex 11 years ago
   You must log in to vote on the helpfulness of this noteVote results for this note: 9You must log in to vote on the helpfulness of this note

   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.

   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.

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2. Skip to note 7 content
   Codex 11 years ago
   You must log in to vote on the helpfulness of this noteVote results for this note: 3You must log in to vote on the helpfulness of this note

   Adding or Updating a Unique Custom Field
   Adds 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' );
   }

   • Just update_post_meta ( 7, 'fruit', 'banana' ); alone does the same.

     Rolf Allard van Hagen 7 years ago
   • In some cases I prefer extra checks like checking if is empty and then delete it. delete_post_meta($post_id, $meta_key, $meta_value);

     capbussat 4 years ago
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3. Skip to note 8 content
   Codex 11 years ago
   You must log in to vote on the helpfulness of this noteVote results for this note: 1You must log in to vote on the helpfulness of this note

   Other Examples
   Adds a new custom field only if a custom field with the given key does not already exists:

   <?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.

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4. Skip to note 9 content
   Codex 11 years ago
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   Default Usage

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

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5. Skip to note 10 content
   capbussat 4 years ago
   You must log in to vote on the helpfulness of this noteVote results for this note: 0You must log in to vote on the helpfulness of this note

   I prefer a more complete solution which checks if is empty and deletes it:

   function my_update_post_meta($post_id, $meta_key, $new_meta_value)
   {
   $meta_value = get_post_meta($post_id, $meta_key, true);
   if ($new_meta_value && ” === $meta_value)
   add_post_meta($post_id, $meta_key, $new_meta_value, true); // unique
   else if ($new_meta_value && $new_meta_value !== $meta_value)
   update_post_meta($post_id, $meta_key, $new_meta_value, $meta_value);
   // same prev_value
   else if (” === $new_meta_value && $meta_value)
   delete_post_meta($post_id, $meta_key, $meta_value); }

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