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Measuring the time of PHP scripts - Using $_SERVER['REQUEST_TIME']

Are this methods a reliable way to measure a script:

$time = ($_SERVER['REQUEST_TIME_FLOAT'] - $_SERVER['REQUEST_TIME']);

or

$time = (microtime(true) - $_SERVER['REQUEST_TIME_FLOAT']);

Which one should be used?

And what's the difference of each one?

They return very different measurements.

Solution
    1. $time = ($_SERVER['REQUEST_TIME_FLOAT'] - $_SERVER['REQUEST_TIME']);

    This will never give you execution time of you PHP script. Because both the values are used for storing start of request. The difference is, $_SERVER['REQUEST_TIME_FLOAT'] is more precise and stores time value with microsecond precision, while $_SERVER['REQUEST_TIME'] in seconds.

    1. $time = (microtime(true) - $_SERVER['REQUEST_TIME_FLOAT']);

    I guess this is what should be used at the end of the PHP script and I think you know why.

    Also keep in mind $_SERVER['REQUEST_TIME_FLOAT'] is available since PHP 5.4.0.