Strings & Patterns Learning Mode

Consider the following script:

<html>
<head>
<title>
       This is a test script.
</title>
</head>
<body>
        <?php
            $string1 = "ab";
            $string2 = "cd";
            $string1 =$string1.$string2;      
           $string3 = "abc";
           $string1 .= $string3;
           echo $string1;
      ?>
</body>
</html>

What will be the output of the above PHP script?