if ($headmod !== 'mainpage') {
echo '<b><div class="phdr"><span class=white>REKLAMA</span><p> </div> <script type="text/javascript" src="http://adsuz.ru/ads1.php"></script></a></span> </p></b>'; 
}

if (isset($_GET['err']) || $headmod = "mainpage" || ($headmod == 'mainpage')) { 
echo '<b><div class="phdr"><span class=white>REKLAMA</span><p> </div> <script type="text/javascript" src="http://adsuz.ru/ads1.php"></script></a></span> </p></b>'; 
}