Index: mbstring.xml
===================================================================
--- mbstring.xml	(revision 0)
+++ mbstring.xml	(working copy)
@@ -0,0 +1,11 @@
+<phpunit
+        bootstrap="bootstrap.php"
+        backupGlobals="false"
+        colors="true"
+        >
+    <testsuites>
+        <testsuite>
+            <directory suffix=".php">tests/mbstring</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>
Index: phpunit.xml
===================================================================
--- phpunit.xml	(revision 1076)
+++ phpunit.xml	(working copy)
@@ -14,6 +14,7 @@
     <groups>
         <exclude>
             <group>ajax</group>
+            <group>mbstring</group>
         </exclude>
     </groups>
     <logging>
Index: tests/mbstring/serialize.php
===================================================================
--- tests/mbstring/serialize.php	(revision 0)
+++ tests/mbstring/serialize.php	(working copy)
@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * @group mbstring
+ */
+class Tests_Mbstring_Serialize extends WP_UnitTestCase {
+
+	/**
+	 * @ticket 18007
+	 */
+	public function test_is_serialized() {
+		/**
+		 * Context
+		 * 
+		 * utf-8 encoded database fields
+		 * auto overloading singlebyte functions from conf (see  http://www.php.net/manual/en/mbstring.overload.php)
+		 */
+
+		if ( ( ini_get( 'mbstring.func_overload' ) & 2 ) == 0 ) {
+			$this->markTestIncomplete( 'Please run as php -d mbstring.func_overload=2 /usr/bin/phpunit -c mbstring.xml');
+		}
+		
+		/**
+		 * Bug scenario
+		 * 
+		 * the strlen call is actually a mb_strlen one
+		 * the $length var is the number of characters (not bytes)
+		 * using $data[$length-1] doesn't retrieve the last char but one before, its actual position is undefined, depending on the other content of the string
+		 * thus $lastc is not ; or }
+		 * the string is understood as not serialized
+		 */
+
+		$data = serialize( 'aéùp' );
+
+		$this->assertTrue( is_serialized( $data ) );
+	}
+}