diff --git a/src/wp-admin/custom-background.php b/src/wp-admin/custom-background.php
index 46a61ccbb1..8b72ab6ac7 100644
--- a/src/wp-admin/custom-background.php
+++ b/src/wp-admin/custom-background.php
@@ -69,6 +69,19 @@ class Custom_Background {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
+
 	/**
 	 * Set up the hooks for the Custom Background admin page.
 	 *
diff --git a/src/wp-admin/custom-header.php b/src/wp-admin/custom-header.php
index 81ff4d6a0b..49025a9e40 100644
--- a/src/wp-admin/custom-header.php
+++ b/src/wp-admin/custom-header.php
@@ -91,6 +91,18 @@ class Custom_Image_Header {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Set up the hooks for the Custom Header admin page.
 	 *
diff --git a/src/wp-admin/includes/class-wp-filesystem-base.php b/src/wp-admin/includes/class-wp-filesystem-base.php
index 86ff7a78db..08eaa0e384 100644
--- a/src/wp-admin/includes/class-wp-filesystem-base.php
+++ b/src/wp-admin/includes/class-wp-filesystem-base.php
@@ -55,6 +55,18 @@ class WP_Filesystem_Base {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Return the path on the remote filesystem of ABSPATH.
 	 *
diff --git a/src/wp-admin/includes/class-wp-list-table.php b/src/wp-admin/includes/class-wp-list-table.php
index 76552d08fe..d4ccfcc7d5 100644
--- a/src/wp-admin/includes/class-wp-list-table.php
+++ b/src/wp-admin/includes/class-wp-list-table.php
@@ -106,6 +106,18 @@ class WP_List_Table {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Make private/protected methods readable for backwards compatibility
 	 *
diff --git a/src/wp-includes/cache.php b/src/wp-includes/cache.php
index 734c968140..ce2cc00f7a 100644
--- a/src/wp-includes/cache.php
+++ b/src/wp-includes/cache.php
@@ -268,7 +268,7 @@ class WP_Object_Cache {
 	 * @access private
 	 * @since 2.0.0
 	 */
-	public $cache = array();
+	private $cache = array();
 
 	/**
 	 * The amount of times the cache data was already stored in the cache.
@@ -317,6 +317,18 @@ class WP_Object_Cache {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Adds data to the cache if it doesn't already exist.
 	 *
diff --git a/src/wp-includes/class-wp-ajax-response.php b/src/wp-includes/class-wp-ajax-response.php
index b32f7aa535..4653a7ca0b 100644
--- a/src/wp-includes/class-wp-ajax-response.php
+++ b/src/wp-includes/class-wp-ajax-response.php
@@ -40,6 +40,18 @@ class WP_Ajax_Response {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Append to XML response based on given arguments.
 	 *
diff --git a/src/wp-includes/class-wp-error.php b/src/wp-includes/class-wp-error.php
index e93d464cc9..7de5812f56 100644
--- a/src/wp-includes/class-wp-error.php
+++ b/src/wp-includes/class-wp-error.php
@@ -75,6 +75,18 @@ class WP_Error {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Retrieve all error codes.
 	 *
diff --git a/src/wp-includes/class-wp-walker.php b/src/wp-includes/class-wp-walker.php
index 7ee5faf1fa..5cb576506f 100644
--- a/src/wp-includes/class-wp-walker.php
+++ b/src/wp-includes/class-wp-walker.php
@@ -50,6 +50,18 @@ class Walker {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Starts the list before the elements are added.
 	 *
diff --git a/src/wp-includes/class-wp.php b/src/wp-includes/class-wp.php
index 2b0590c99a..91dc2348d8 100644
--- a/src/wp-includes/class-wp.php
+++ b/src/wp-includes/class-wp.php
@@ -673,6 +673,18 @@ class WP_MatchesMapRegex {
 		return $this->$name;
 	}
 
+	/**
+	 * Make private properties setable for backwards compatibility
+	 *
+	 * @since 4.0.0
+	 * @param string $name
+	 * @param string $value
+	 * @return mixed
+	 */
+	public function __set( $name, $value ) {
+		return $this->$name = $value;
+	}
+
 	/**
 	 * Make private/protected methods readable for backwards compatibility
 	 *