I don't know about getting precise CPU time data, but you can easily check how long PHP takes to process a page:
Straight from the PHP manual:
<?php
function getmicrotime(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);
}
$time_start = getmicrotime();
for ($i=0; $i < 1000; $i++){
//do nothing, 1000 times
}
$time_end = getmicrotime();
$time = $time_end - $time_start;
echo "Did nothing in $time seconds";
?>