Accurate way to measure execution times of php scripts

You can use the microtime function for this. From the documentation:

microtime — Return current Unix timestamp with microseconds

---

If get_as_float is set to TRUE, then microtime() returns a float, which represents the current time in seconds since the Unix epoch accurate to the nearest microsecond.

Example usage:

$start = microtime(true);
while (...) {

}
$time_elapsed_secs = microtime(true) - $start;