#header-1 {background:url(../random/header1.jpg) no-repeat;#header-2 {background:url(../random/header2.jpg) no-repeat;#header-3 {background:url(../random/header3.jpg) no-repeat;#header-4 {background:url(../random/header4.jpg) no-repeat;
<?php $h=rand(1,4); ?> <div id="header-<?php print $h;?>"></div>