PHP: Stopwatch function to time pieces of code

I need to time how long a certain piece of code takes to complete. I wrote a simple PHP stopwatch function to accomplish this:

sw(); // Start the stopwatch
$x  = some_slow_code();
$ms = sw(); // Return milliseconds since start
// Stopwatch function: returns milliseconds
function sw() {
    static $start = null;

    if (!$start) {
        $start = hrtime(1);
    } else {
        $ret   = (hrtime(1) - $start) / 1000000;
        $start = null; // Reset the start time
        return $ret;

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