$time_start = microtime(); //Ставится в начале скрипта.
//В конце скрипта
$time_format = explode(' ', $time_start);
$time_start = $time_format[1] + $time_format[0];
$time_end = microtime();
$time_format = explode(' ', $time_end);
$time_end = $time_format[1] + $time_format[0];
$rrr = $time_end - $time_start;
echo $rrr;