why is the iframe contents empty?

<html>
...
<body>
  <table>
     <tr>
       <td>
         User info: <iframe id="SpControlFrame1" name="SpControlFrame1" src="javascript:'';"path_src="index.php?cmd=YYY" ></iframe>
        </td>
     </tr>
   </table>
 </body>
 </html>

switch ("cmd") {
 case ZZZ:
   include("foo.php");
   break;
 case YYY:
   include("blah.php")
   break;
}

<?php
 //some funtions for processing
?>
<html>
 <head>
  ...
 </head>
 <body>
   <input type="text" size="12" name="username">
   <input type="button" value="submit">
  </body>
</html>

User info: <iframe id="SpControlFrame1" name="SpControlFrame1" src="index.php?cmd=YYY" ></iframe>