计算程序执行的时间，可以作为写代码的参考

$time_start = microtime(true);
$a=array('111','222','333','222','333','222','333','222','333','222','333','222','333','222','333','222','333','222','333','222','333','222','333','222','333','222','333');
for($i=1;$i<count($a);$i++){
 $a[$i];
}
$time_end=microtime(true);
$time=$time_end-$time_start;
echo number_format($time, 10, '.','')." seconds";//0.0000679493 seconds