Close Browser Window

PHP is executed at the server-side and thus does not know about clients, browsers or windows of those browsers.

Some options though:

Close button
<?php
  echo "<BUTTON onclick='window.close();'>CLOSE</BUTTON>";

  <a onclick='window.close();' style='cursor:pointer;' >Close</a>
?>
Close using java script
<script language=JavaScript>
window.close();
</script> 
<a class="GoBackButton" href="javascript:window.close();"><div class="GoBackButton">Go Back</div></a>