//Create a variable for start time
$time_start = microtime(true);

// Place your PHP/HTML/JavaScript/CSS/Etc. Here

//Create a variable for end time
$time_end = microtime(true);
//Subtract the two times to get seconds
$time = $time_end - $time_start;

echo 'Script took '.$time.' seconds to execute';

http://phpsnips.com/snippet.php?id=26

Calculate execution time

PHP