Bad Boolean external represenation *HELP*!

Hi

Not sure if this belongs in novice or somewhere a bit 'higher' but I'll
stick it here anyway.

I'm running a series of PHP scripts through postgre 7.3.4 at college to
create an online portfolio creator (if that makes sense)

Anyway, this piece of code is throwing out Warning: pg_query(): Query
failed: ERROR: Bad boolean external representation 'testuser2' in
xxxxxxxxx/task.php on line 90

Here is the segment that the error is reporting:

if ($GroupTask == 'No')
        {
        print("Error: This task is not set as a group task.<br/>
              To access this task the task creator must alter the task settings.<br/>
              Click <a href=\"taskmain.php\">here</a> to return to the main task
page.");
        }

        else
        {
        print("<a href=\"journal.php\">Journal</a>
               <a href=\"schedule.php\">Schedule</a>
               <a href=\"document.php\">Documents</a>
               <a href=\"links.php\">Links</a>
               <a href=\"contacts.php\">Contacts</a><br/><br/>
               <h2>$GroupTaskName</h2><br/>
               This task created by <strong>$GroupTaskCreatedBy</strong><br/><br/>
               Hello $Username<br/>
               This task was last accessed on $LastAccessed by $LastEdited");

        $UpdateQuery = "UPDATE tasks SET LastEdited = '$Username' AND LastAccessed
= '$timestamp' WHERE UniqueID = '$UniqueGroupID' and TaskID =
'$GroupTaskID';";

    Line 90-->    $UpdateQueryExecute = pg_query($DBconnection, $UpdateQuery);

I cannot understand why it is throwing it back, particularly as I'm running
virtually the exact same update query elsewhere in the same script :|

Any help or advice HUGELY appreciated.

Adam