Ticket #43941: 43941.2.diff

src/wp-includes/meta.php

@@ -532 +532 @@
                         return array_map( 'maybe_unserialize', $meta_cache[ $meta_key ] );
                 }
         }
-
-        if ( $single ) {
-                return '';
-        } else {
-                return array();
+        $object_subtype = get_object_subtype( $meta_type, $object_id );
+        $default        = get_metadata_default( $meta_type, $meta_key, $object_subtype );
+        if ( ! $single ) {
+                $default_value = (array) $default_value;
         }
+        return $default;
 }
 
 /**
@@ -1070 +1070 @@
                 'sanitize_callback' => null,
                 'auth_callback'     => null,
                 'show_in_rest'      => false,
+                'default_value'     => false,
         );
 
         // There used to be individual args for sanitize and auth callbacks
@@ -1145 +1146 @@
 }
 
 /**
+ * Get default value for meta
+ *
+ * @param $object_type
+ * @param $meta_key
+ * @param string $object_subtype
+ *
+ * @return mixed|void
+ */
+function get_metadata_default( $object_type, $meta_key, $object_subtype = '' ) {
+        global $wp_meta_keys;
+
+        if ( ! is_array( $wp_meta_keys ) ) {
+                $wp_meta_keys = array();
+        }
+
+        $default_value = '';
+
+        if ( isset( $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ]['default_value'] ) &&
+             false !== $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ]['default_value'] ) {
+                $default_value = $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ]['default_value'];
+        }
+
+        /**
+         *
+         * @since 5.0.0
+         *
+         * @param null|array|string $default_value The default value of get_metadata() should return - a single metadata value,
+         *                                     or an array of values.
+         * @param $meta_key
+         * @param $object_type
+         * @param string $object_subtype.
+         */
+        $default_value = apply_filters( "default_{$object_type}_meta_{$meta_key}", $default_value, $meta_key, $object_type, $object_subtype );
+
+        if ( ! empty( $object_subtype ) ) {
+                /**
+                 *
+                 * @since 5.0.0
+                 *
+                 * @param null|array|string $default_value The default value of get_metadata() should return - a single metadata value,
+                 *                                     or an array of values.
+                 * @param $meta_key
+                 * @param $object_type
+                 * @param string $object_subtype.
+                 */
+                $default_value = apply_filters( "default_{$object_type}_meta_{$meta_key}_for_{$object_subtype}", $default_value, $meta_key, $object_type, $object_subtype );
+        }
+
+
+        return $default_value;
+}
+
+/**
  * Checks if a meta key is registered.
  *
  * @since 4.6.0