get_post_meta()　WordPress関数　テンプレートタグ

<?php

//wp-includes/post.phpより

/**
 * Retrieves a post meta field for the given post ID.
 *
 * @since 1.5.0
 *
 * @param int    $post_id Post ID.
 * @param string $key     Optional. The meta key to retrieve. By default, returns
 *                        data for all keys. Default empty.
 * @param bool   $single  Optional. If true, returns only the first value for the specified meta key.
 *                        This parameter has no effect if $key is not specified. Default false.
 * @return mixed Will be an array if $single is false. Will be value of the meta
 *               field if $single is true.
 */
function get_post_meta( $post_id, $key = '', $single = false ) {
	return get_metadata( 'post', $post_id, $key, $single );
}

//wp-includes/meta.phpより

function get_metadata( $meta_type, $object_id, $meta_key = '', $single = false ) {
	if ( ! $meta_type || ! is_numeric( $object_id ) ) {
		return false;
	}

	$object_id = absint( $object_id );
	if ( ! $object_id ) {
		return false;
	}

	/**
	 * Filters whether to retrieve 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.
	 *
	 * @since 3.1.0
	 *
	 * @param null|array|string $value     The value get_metadata() should return - a single metadata value,
	 *                                     or an array of values.
	 * @param int               $object_id ID of the object metadata is for.
	 * @param string            $meta_key  Metadata key.
	 * @param bool              $single    Whether to return only the first value of the specified $meta_key.
	 */
	$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, $single );
	if ( null !== $check ) {
		if ( $single && is_array( $check ) ) {
			return $check[0];
		} else {
			return $check;
		}
	}

	$meta_cache = wp_cache_get( $object_id, $meta_type . '_meta' );

	if ( ! $meta_cache ) {
		$meta_cache = update_meta_cache( $meta_type, array( $object_id ) );
		if ( isset( $meta_cache[ $object_id ] ) ) {
			$meta_cache = $meta_cache[ $object_id ];
		} else {
			$meta_cache = null;
		}
	}

	if ( ! $meta_key ) {
		return $meta_cache;
	}

	if ( isset( $meta_cache[ $meta_key ] ) ) {
		if ( $single ) {
			return maybe_unserialize( $meta_cache[ $meta_key ][0] );
		} else {
			return array_map( 'maybe_unserialize', $meta_cache[ $meta_key ] );
		}
	}

	if ( $single ) {
		return '';
	} else {
		return array();
	}
}
?>