diff --git a/wp-admin/admin.php b/wp-admin/admin.php
index d9335c521e..7941d26a35 100644
--- a/wp-admin/admin.php
+++ b/wp-admin/admin.php
@@ -95,26 +95,24 @@ wp_enqueue_script( 'common' );
 //     declared as globals here
 global $pagenow, $hook_suffix, $plugin_page, $typenow, $taxnow;
 
-$page_hook = '';
-$hook_suffix = '';
-$plugin_page = '';
-$typenow = '';
-$taxnow = '';
+$page_hook = null;
 
 $editing = false;
 
-if ( isset( $_GET['page'] ) ) {
+if ( isset($_GET['page']) ) {
 	$plugin_page = wp_unslash( $_GET['page'] );
-	$plugin_page = plugin_basename( $plugin_page );
+	$plugin_page = plugin_basename($plugin_page);
 }
 
-if ( isset( $_REQUEST['post_type'] ) && post_type_exists( $_REQUEST['post_type'] ) ) {
+if ( isset( $_REQUEST['post_type'] ) && post_type_exists( $_REQUEST['post_type'] ) )
 	$typenow = $_REQUEST['post_type'];
-}
+else
+	$typenow = '';
 
-if ( isset( $_REQUEST['taxonomy'] ) && taxonomy_exists( $_REQUEST['taxonomy'] ) ) {
+if ( isset( $_REQUEST['taxonomy'] ) && taxonomy_exists( $_REQUEST['taxonomy'] ) )
 	$taxnow = $_REQUEST['taxonomy'];
-}
+else
+	$taxnow = '';
 
 if ( WP_NETWORK_ADMIN )
 	require(ABSPATH . 'wp-admin/network/menu.php');
@@ -153,12 +151,11 @@ if ( current_user_can( 'manage_options' ) ) {
  */
 do_action( 'admin_init' );
 
-if ( $plugin_page ) {
-	if ( $typenow ) {
+if ( isset($plugin_page) ) {
+	if ( !empty($typenow) )
 		$the_parent = $pagenow . '?post_type=' . $typenow;
-	} else {
+	else
 		$the_parent = $pagenow;
-	}
 	if ( ! $page_hook = get_plugin_page_hook($plugin_page, $the_parent) ) {
 		$page_hook = get_plugin_page_hook($plugin_page, $plugin_page);
 
@@ -176,18 +173,19 @@ if ( $plugin_page ) {
 	unset($the_parent);
 }
 
-if ( $page_hook ) {
+$hook_suffix = '';
+if ( isset( $page_hook ) ) {
 	$hook_suffix = $page_hook;
-} elseif ( $plugin_page ) {
+} elseif ( isset( $plugin_page ) ) {
 	$hook_suffix = $plugin_page;
-} elseif ( $pagenow ) {
+} elseif ( isset( $pagenow ) ) {
 	$hook_suffix = $pagenow;
 }
 
 set_current_screen();
 
 // Handle plugin admin pages.
-if ( $plugin_page ) {
+if ( isset($plugin_page) ) {
 	if ( $page_hook ) {
 		/**
 		 * Fires before a particular screen is loaded.
diff --git a/wp-admin/includes/plugin.php b/wp-admin/includes/plugin.php
index 3c2cf0007b..c0d44ff6fc 100644
--- a/wp-admin/includes/plugin.php
+++ b/wp-admin/includes/plugin.php
@@ -1508,7 +1508,7 @@ function get_admin_page_parent( $parent = '' ) {
 		return $parent;
 	}
 
-	if ( $pagenow == 'admin.php' && $plugin_page ) {
+	if ( $pagenow == 'admin.php' && isset( $plugin_page ) ) {
 		foreach ( (array)$menu as $parent_menu ) {
 			if ( $parent_menu[2] == $plugin_page ) {
 				$parent_file = $plugin_page;
@@ -1525,7 +1525,7 @@ function get_admin_page_parent( $parent = '' ) {
 		}
 	}
 
-	if ( $plugin_page && isset( $_wp_submenu_nopriv[$pagenow][$plugin_page] ) ) {
+	if ( isset( $plugin_page ) && isset( $_wp_submenu_nopriv[$pagenow][$plugin_page] ) ) {
 		$parent_file = $pagenow;
 		if ( isset( $_wp_real_parent_file[$parent_file] ) )
 			$parent_file = $_wp_real_parent_file[$parent_file];
@@ -1536,13 +1536,13 @@ function get_admin_page_parent( $parent = '' ) {
 		foreach ( $submenu[$parent] as $submenu_array ) {
 			if ( isset( $_wp_real_parent_file[$parent] ) )
 				$parent = $_wp_real_parent_file[$parent];
-			if ( $typenow && ($submenu_array[2] == "$pagenow?post_type=$typenow") ) {
+			if ( !empty($typenow) && ($submenu_array[2] == "$pagenow?post_type=$typenow") ) {
 				$parent_file = $parent;
 				return $parent;
-			} elseif ( $submenu_array[2] == $pagenow && ! $typenow && ( empty($parent_file) || false === strpos($parent_file, '?') ) ) {
+			} elseif ( $submenu_array[2] == $pagenow && empty($typenow) && ( empty($parent_file) || false === strpos($parent_file, '?') ) ) {
 				$parent_file = $parent;
 				return $parent;
-			} elseif ( $plugin_page && ($plugin_page == $submenu_array[2] ) ) {
+			} elseif ( isset( $plugin_page ) && ($plugin_page == $submenu_array[2] ) ) {
 				$parent_file = $parent;
 				return $parent;
 			}
@@ -1575,7 +1575,7 @@ function get_admin_page_title() {
 				if ( $menu_array[2] == $pagenow ) {
 					$title = $menu_array[3];
 					return $menu_array[3];
-				} elseif ( $plugin_page && ($plugin_page == $menu_array[2] ) && ($hook == $menu_array[3] ) ) {
+				} elseif ( isset( $plugin_page ) && ($plugin_page == $menu_array[2] ) && ($hook == $menu_array[3] ) ) {
 					$title = $menu_array[3];
 					return $menu_array[3];
 				}
@@ -1587,14 +1587,14 @@ function get_admin_page_title() {
 	} else {
 		foreach ( array_keys( $submenu ) as $parent ) {
 			foreach ( $submenu[$parent] as $submenu_array ) {
-				if ( $plugin_page &&
+				if ( isset( $plugin_page ) &&
 					( $plugin_page == $submenu_array[2] ) &&
 					(
 						( $parent == $pagenow ) ||
 						( $parent == $plugin_page ) ||
 						( $plugin_page == $hook ) ||
 						( $pagenow == 'admin.php' && $parent1 != $submenu_array[2] ) ||
-						( $typenow && $parent == $pagenow . '?post_type=' . $typenow)
+						( !empty($typenow) && $parent == $pagenow . '?post_type=' . $typenow)
 					)
 					) {
 						$title = $submenu_array[3];
@@ -1615,7 +1615,7 @@ function get_admin_page_title() {
 		}
 		if ( empty ( $title ) ) {
 			foreach ( $menu as $menu_array ) {
-				if ( $plugin_page &&
+				if ( isset( $plugin_page ) &&
 					( $plugin_page == $menu_array[2] ) &&
 					( $pagenow == 'admin.php' ) &&
 					( $parent1 == $menu_array[2] ) )
@@ -1670,10 +1670,10 @@ function user_can_access_admin_page() {
 
 	$parent = get_admin_page_parent();
 
-	if ( ! $plugin_page && isset( $_wp_submenu_nopriv[$parent][$pagenow] ) )
+	if ( !isset( $plugin_page ) && isset( $_wp_submenu_nopriv[$parent][$pagenow] ) )
 		return false;
 
-	if ( $plugin_page ) {
+	if ( isset( $plugin_page ) ) {
 		if ( isset( $_wp_submenu_nopriv[$parent][$plugin_page] ) )
 			return false;
 
@@ -1688,25 +1688,25 @@ function user_can_access_admin_page() {
 			return false;
 		if ( isset( $_wp_submenu_nopriv[$pagenow][$pagenow] ) )
 			return false;
-		if ( $plugin_page && isset( $_wp_submenu_nopriv[$pagenow][$plugin_page] ) )
+		if ( isset( $plugin_page ) && isset( $_wp_submenu_nopriv[$pagenow][$plugin_page] ) )
 			return false;
-		if ( $plugin_page && isset( $_wp_menu_nopriv[$plugin_page] ) )
+		if ( isset( $plugin_page ) && isset( $_wp_menu_nopriv[$plugin_page] ) )
 			return false;
 		foreach (array_keys( $_wp_submenu_nopriv ) as $key ) {
 			if ( isset( $_wp_submenu_nopriv[$key][$pagenow] ) )
 				return false;
-			if ( $plugin_page && isset( $_wp_submenu_nopriv[$key][$plugin_page] ) )
+			if ( isset( $plugin_page ) && isset( $_wp_submenu_nopriv[$key][$plugin_page] ) )
 			return false;
 		}
 		return true;
 	}
 
-	if ( $plugin_page && ( $plugin_page == $parent ) && isset( $_wp_menu_nopriv[$plugin_page] ) )
+	if ( isset( $plugin_page ) && ( $plugin_page == $parent ) && isset( $_wp_menu_nopriv[$plugin_page] ) )
 		return false;
 
 	if ( isset( $submenu[$parent] ) ) {
 		foreach ( $submenu[$parent] as $submenu_array ) {
-			if ( $plugin_page && ( $submenu_array[2] == $plugin_page ) ) {
+			if ( isset( $plugin_page ) && ( $submenu_array[2] == $plugin_page ) ) {
 				if ( current_user_can( $submenu_array[1] ))
 					return true;
 				else
diff --git a/wp-admin/menu-header.php b/wp-admin/menu-header.php
index 1279056dc4..c599fd1be8 100644
--- a/wp-admin/menu-header.php
+++ b/wp-admin/menu-header.php
@@ -66,7 +66,7 @@ function _wp_menu_output( $menu, $submenu, $submenu_as_parent = true ) {
 			$submenu_items = $submenu[$item[2]];
 		}
 
-		if ( ( $parent_file && $item[2] == $parent_file ) || ( ! $typenow && $self == $item[2] ) ) {
+		if ( ( $parent_file && $item[2] == $parent_file ) || ( empty($typenow) && $self == $item[2] ) ) {
 			$class[] = ! empty( $submenu_items ) ? 'wp-has-current-submenu wp-menu-open' : 'current';
 		} else {
 			$class[] = 'wp-not-current-submenu';
@@ -167,7 +167,7 @@ function _wp_menu_output( $menu, $submenu, $submenu_as_parent = true ) {
 					$menu_file = substr( $menu_file, 0, $pos );
 
 				// Handle current for post_type=post|page|foo pages, which won't match $self.
-				$self_type = $typenow ? $self . '?post_type=' . $typenow : 'nothing';
+				$self_type = ! empty( $typenow ) ? $self . '?post_type=' . $typenow : 'nothing';
 
 				if ( isset( $submenu_file ) ) {
 					if ( $submenu_file == $sub_item[2] )
@@ -175,8 +175,8 @@ function _wp_menu_output( $menu, $submenu, $submenu_as_parent = true ) {
 				// If plugin_page is set the parent must either match the current page or not physically exist.
 				// This allows plugin pages with the same hook to exist under different parents.
 				} elseif (
-					( ! $plugin_page && $self == $sub_item[2] ) ||
-					( $plugin_page && $plugin_page == $sub_item[2] && ( $item[2] == $self_type || $item[2] == $self || file_exists($menu_file) === false ) )
+					( ! isset( $plugin_page ) && $self == $sub_item[2] ) ||
+					( isset( $plugin_page ) && $plugin_page == $sub_item[2] && ( $item[2] == $self_type || $item[2] == $self || file_exists($menu_file) === false ) )
 				) {
 					$class[] = 'current';
 				}
diff --git a/wp-includes/deprecated.php b/wp-includes/deprecated.php
index f97e13ca5f..c3d2e04e05 100644
--- a/wp-includes/deprecated.php
+++ b/wp-includes/deprecated.php
@@ -2576,7 +2576,7 @@ function is_plugin_page() {
 
 	global $plugin_page;
 
-	if ( $plugin_page )
+	if ( isset($plugin_page) )
 		return true;
 
 	return false;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 5d4cd0cb70..4cf619faa4 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31105';
+$wp_version = '4.2-alpha-31106';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.