PHP计算执行时间，有时返回负值

Question

使用下面的脚本计算执行时间:

$ php -r "\$tt=microtime();for(\$i=0;\$i<111120;\$i++) \
  echo hash('crc32', microtime()).PHP_EOL;echo 'Time took: ' \
   . (microtime()-\$tt).PHP_EOL;"

结果(多个):

...
Time took: 0.266269
...
Time took: -0.725037
...
Time took: 0.264577
...
Time took: 0.655573
...
Time took: -0.389367
...
Time took: -0.451503
...
Time took: 0.50867

为什么时间计算有时会返回负值？

Answer 1

传递第二个参数:

microtime(true);

否则你会得到一个字符串而不是一个真正的浮点值，这会导致意想不到的/错误的结果:

By default, microtime() returns a string in the form "msec sec", where sec is the current time measured in the number of seconds since the Unix epoch (0:00:00 January 1, 1970 GMT), and msec is the number of microseconds that have elapsed since sec expressed in seconds.

— php.net/microtime

(强调我的)

例子:

microtime():     0.93146600 1382611111   (string value)
microtime(true): 1382611111.93146600     (float, printed as a string in this case)