This code is based on the ASP script posted by Lio. It will determine the time taken for a php script to execute correct to 0.000000000000001 seconds.
<!-- put this at the top of the page -->
<?php
$mtime = microtime();
$mtime = explode(" ",$mtime);
$mtime = $mtime[1] + $mtime[0];
$starttime = $mtime;
;?>
<!-- put other code and html in here -->
<!-- put this code at the bottom of the page -->
<?php
$mtime = microtime();
$mtime = explode(" ",$mtime);
$mtime = $mtime[1] + $mtime[0];
$endtime = $mtime;
$totaltime = ($endtime - $starttime);
echo "This page was created in ".$totaltime." seconds";
;?>