<?php

$imagearrdisplay = ",8,7,22,";

$displayitem = explode(",", trim($imagearrdisplay, ','));

shuffle($displayitem);

$class = 'firstbanner';
foreach($displayitem as $item)
{
   $src = sprintf('/images/transitions/%d.jpg', $item);
   printf("<div class=\"%s\">\n  <img src=\"%s\">\n</div>\n", $class, $src);
   $class = 'banner';
}

<div class="firstbanner">
  <img src="/images/transitions/7.jpg">
</div>
<div class="banner">
  <img src="/images/transitions/22.jpg">
</div>
<div class="banner">
  <img src="/images/transitions/8.jpg">
</div>