On many sites you must have noticed a message at the bootom of the page which looks something like "This page was created in 0.2365 seconds".

PHP Code
At the top of the PHP script put the following piece of code.

<?php
   $mtime = microtime();
   $mtime = explode(" ",$mtime);
   $mtime = $mtime[1] + $mtime[0];
   $starttime = $mtime;
?>

And put the following code at the bottom and you are done.

<?php
   $mtime = microtime();
   $mtime = explode(" ",$mtime);
   $mtime = $mtime[1] + $mtime[0];
   $endtime = $mtime;
   $totaltime = ($endtime - $starttime);
   echo "This page was created in ".$totaltime." seconds";
?>