diff --git src/wp-includes/class-wp-widget-factory.php src/wp-includes/class-wp-widget-factory.php
index 9b9214e..ce37708 100644
--- src/wp-includes/class-wp-widget-factory.php
+++ src/wp-includes/class-wp-widget-factory.php
@@ -46,6 +46,50 @@ class WP_Widget_Factory {
 	}
 
 	/**
+	 * Memory for the number of times unique class instances have been hashed.
+	 *
+	 * This can be eliminated in favor of straight spl_object_hash() when 5.3
+	 * is the minimum requirement for PHP.
+	 *
+	 * @since 4.6.0
+	 * @access private
+	 * @see WP_Widget_Factory::hash_object()
+	 *
+	 * @var array
+	 */
+	private $hashed_class_counts = array();
+
+	/**
+	 * Hash an object, doing fallback of `spl_object_hash()` if not available.
+	 *
+	 * This can be eliminated in favor of straight spl_object_hash() when 5.3
+	 * is the minimum requirement for PHP.
+	 *
+	 * @since 4.6.0
+	 * @access private
+	 *
+	 * @param WP_Widget $widget Widget.
+	 * @return string
+	 */
+	private function hash_object( $widget ) {
+		if ( function_exists( 'spl_object_hash' ) ) {
+			return spl_object_hash( $widget );
+		} else {
+			$class_name = get_class( $widget );
+			$hash = $class_name;
+			if ( ! isset( $widget->_wp_widget_factory_hash_id ) ) {
+				if ( ! isset( $this->hashed_class_counts[ $class_name ] ) ) {
+					$this->hashed_class_counts[ $class_name ] = 0;
+				}
+				$this->hashed_class_counts[ $class_name ] += 1;
+				$widget->_wp_widget_factory_hash_id = $this->hashed_class_counts[ $class_name ];
+			}
+			$hash .= ':' . $widget->_wp_widget_factory_hash_id;
+			return $hash;
+		}
+	}
+
+	/**
 	 * Registers a widget subclass.
 	 *
 	 * @since 2.8.0
@@ -56,7 +100,7 @@ class WP_Widget_Factory {
 	 */
 	public function register( $widget ) {
 		if ( $widget instanceof WP_Widget ) {
-			$this->widgets[ spl_object_hash( $widget ) ] = $widget;
+			$this->widgets[ $this->hash_object( $widget ) ] = $widget;
 		} else {
 			$this->widgets[ $widget ] = new $widget();
 		}
@@ -73,7 +117,7 @@ class WP_Widget_Factory {
 	 */
 	public function unregister( $widget ) {
 		if ( $widget instanceof WP_Widget ) {
-			unset( $this->widgets[ spl_object_hash( $widget ) ] );
+			unset( $this->widgets[ $this->hash_object( $widget ) ] );
 		} else {
 			unset( $this->widgets[ $widget ] );
 		}
diff --git tests/phpunit/tests/widgets.php tests/phpunit/tests/widgets.php
index 0d0234c..d0fe7ca 100644
--- tests/phpunit/tests/widgets.php
+++ tests/phpunit/tests/widgets.php
@@ -78,7 +78,7 @@ class Tests_Widgets extends WP_UnitTestCase {
 		$widget_better_search->widget_options['classname'] = 'widget_' . $widget_better_search->id_base;
 		$widget_better_search->control_options['id_base'] = $widget_better_search->id_base;
 		register_widget( $widget_better_search );
-		$this->assertArrayHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
+		$this->assertContains( $widget_better_search, $wp_widget_factory->widgets );
 
 		$widget_best_search = new WP_Widget_Search();
 		$widget_best_search->id_base = 'best_search';
@@ -87,12 +87,12 @@ class Tests_Widgets extends WP_UnitTestCase {
 		$widget_best_search->widget_options['classname'] = 'widget_' . $widget_best_search->id_base;
 		$widget_best_search->control_options['id_base'] = $widget_best_search->id_base;
 		register_widget( $widget_best_search );
-		$this->assertArrayHasKey( spl_object_hash( $widget_best_search ), $wp_widget_factory->widgets );
+		$this->assertContains( $widget_best_search, $wp_widget_factory->widgets );
 
 		$this->assertCount( 3, $wp_widget_factory->widgets );
 		$this->assertArrayHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
-		$this->assertArrayHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
-		$this->assertArrayHasKey( spl_object_hash( $widget_best_search ), $wp_widget_factory->widgets );
+		$this->assertContains( $widget_better_search, $wp_widget_factory->widgets );
+		$this->assertContains( $widget_best_search, $wp_widget_factory->widgets );
 
 		$wp_widget_factory->_register_widgets();
 
@@ -103,14 +103,14 @@ class Tests_Widgets extends WP_UnitTestCase {
 		$this->assertSame( $widget_better_search, $wp_registered_widgets['better_search-3']['callback'][0] );
 		$this->assertSame( $widget_best_search, $wp_registered_widgets['best_search-4']['callback'][0] );
 
-		$this->assertArrayHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
-		$this->assertArrayHasKey( spl_object_hash( $widget_best_search ), $wp_widget_factory->widgets );
+		$this->assertContains( $widget_better_search, $wp_widget_factory->widgets );
+		$this->assertContains( $widget_best_search, $wp_widget_factory->widgets );
 		$this->assertArrayHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
 		unregister_widget( 'WP_Widget_Search' );
 		unregister_widget( $widget_better_search );
 		unregister_widget( $widget_best_search );
-		$this->assertArrayNotHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
-		$this->assertArrayNotHasKey( spl_object_hash( $widget_best_search ), $wp_widget_factory->widgets );
+		$this->assertNotContains( $widget_better_search, $wp_widget_factory->widgets );
+		$this->assertNotContains( $widget_best_search, $wp_widget_factory->widgets );
 		$this->assertArrayNotHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
 	}