Tiempo de ejecucion en PHP

Para saber el tiempo que tarda en ejecutar una función / script / tarea en PHP y ver dónde se produce el mayor gasto de tiempo, se puede usar:

<!-- 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";
;?>


Sacado de http://www.developerfusion.com/code/2058/determine-execution-time-in-php/