Validator get's different html from what I see

From: Ernest Schloesser <ernest@lochmuehle.net> Date: Sun, 30 Dec 2007 01:05:03 +0100 Message-ID: <4776E0AF.3010703@lochmuehle.net> To: www-validator@w3.org · This archive was generated by hypermail 2.4.0 : Friday, 17 January 2020 22:59:05 UTC

Hello,

here is my problem. The website was valid for a long time. Suddenly, all 
links show errors.
As all programmers do, I also claim: I changed nothing!
What I found is that the source code as displayed by the validator is 
different from the source-code that I see in firefox.

Here is a snippet of what the validator sees:
# <body>
# <div class="topmenu"><table><tr>
# <td style="width:9.6em;"><a 
href="index.php?PHPSESSID=0af81ccd17c3e6deba81ef66ebac5c2b">Home</a> 
&nbsp;</td>
# <td style="width:9.6em;"><a 
href="ueberuns.php?PHPSESSID=0af81ccd17c3e6deba81ef66ebac5c2b">Über 
uns</a> &nbsp;</td>

Here is what I see in firefox:
<body>
<div class="topmenu"><table><tr>
<td style="width:9.6em;"><a href="index.php">Home</a> &nbsp;</td>
<td style="width:9.6em;"><a href="ueberuns.php">Über uns</a> &nbsp;</td>

My PHP code that produces this snippet:
function MB_Layout()
{
   echo "\n<body>\n";
   echo '<div class="topmenu">';
   echo '<table>';
   echo '<tr>';
   echo "\n";
   echo '<td style="width:9.6em;"><a href="index.php">Home</a> &nbsp;</td>';
   echo "\n";
   echo '<td style="width:9.6em;"><a href="ueberuns.php">Über uns</a> 
&nbsp;</td>';
   echo "\n";

The url of the website that I have the problem with:
www.dasmassband.de

where do these sessionIDs come from?

Please help!!

Greetings.

Ernest Schlösser