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filter is not described

add_(meta_type)_metadata filter-hook . WP 3.1.0

Filters whether to add metadata of a specific type.

The dynamic portion of the hook, $meta_type, refers to the meta object type (comment, post, term, or user). Returning a non-null value will effectively short-circuit the function.

Usage

add_filter( 'add_(meta_type)_metadata', 'filter_function_name_2515', 10, 5 );
function filter_function_name_2515( $check, $object_id, $meta_key, $meta_value, $unique ){
	// filter...

	return $check;
}
$check(null/true/false)
Whether to allow adding metadata for the given type.
$object_id(int)
Object ID.
$meta_key(string)
Meta key.
$meta_value(mixed)
Meta value. Must be serializable if non-scalar.
$unique(true/false)
Whether the specified meta key should be unique for the object. Optional.
Default: false

Where the hook is called

add_metadata()
wp-includes/meta.php 72
$check = apply_filters( "add_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $unique );

Where the hook is used (in WP core)

wp-includes/default-filters.php 102
add_filter( 'add_term_metadata', 'wp_check_term_meta_support_prefilter' );