Problem solved... Moved the calculation into the start of the loop area and defined the sum as a variable ($days) then created a line to just show that value.
while(!$rsSummaryOpen->atEnd()) {
$now = time();
$invoiceDate = strtotime($rsSummaryOpen->getColumnVal("invDate"));
$difference = $now - $invoiceDate;
$days = floor($difference/(60*60*24));
$MailBody = $MailBody . "\r\n";
$MailBody = $MailBody . " <tr>\r\n";
$MailBody = $MailBody . " <td>";
$MailBody = $MailBody . ($rsSummaryOpen->getColumnVal("invDate"));
$MailBody = $MailBody . "</td>\r\n";
$MailBody = $MailBody . " <td>";
$MailBody = $MailBody . ($rsSummaryOpen->getColumnVal("invoiceNo"));
$MailBody = $MailBody . "</td>\r\n";
$MailBody = $MailBody . " <td><div align=\"right\">$ ";
$MailBody = $MailBody . number_format($rsSummaryOpen->getColumnVal("invValue"), $decimals = 2 , $dec_point = '.' , $thousands_sep = ',' );
$MailBody = $MailBody . "</div></td>\r\n";
$MailBody = $MailBody . " <td><div align=\"right\">\r\n";
$MailBody = $MailBody . "\r\n";
$MailBody = $MailBody . $days;
$MailBody = $MailBody . "\r\n";
$MailBody = $MailBody . " </div></td>\r\n";
$MailBody = $MailBody . " </tr>\r\n";
$MailBody = $MailBody . " ";