Changing to PHP is definitely recommended, Microsoft abandoned classic ASP in about 2000 in favor of .NET with VB.Net and C#. Theres no telling how much longer classic ASP will be still be supported on Windows Web Servers.
your if statement in php would be:
<?php if((isset($_GET['querystringname']))?$_GET['querystringname']:"" == "1" { ?>
HTML code here
<?php } ?>
there really isn't a one to one conversion between PHP and ASP, PHP is more of a pure object oriented language than ASP is, ASP is more of a scripting language.