<?php

 $num = 8.16789;

 //第一种：利用round()对浮点数进行四舍五入

 echo round($num,2).PHP_EOL; //8.17

 //第二种：利用sprintf格式化字符串

 $format_num = sprintf("%.2f",$num);

 echo $format_num.PHP_EOL; //8.17

 //第三种：利用千位分组来格式化数字的函数number_format()

 echo number_format($num, 2).PHP_EOL; //8.17

 //或者如下

 echo number_format($num, 2, '.', '').PHP_EOL; //8.17
15 //或者如下
16 echo number_format(2,2);//2.00 以千位分隔符方式格式化一个数字 不夠自動補0

 number_format(number,decimals,decimalpoint,separator)

 <?php

 $a = 10;

 echo number_format($a, '2') . "<br/>";

 $b = 100000;

 echo number_format($b, '2') . "<br/>";

 $c = 5458.5684;

 echo number_format($c, '2') . "<br/>";

 $d = '1254.8963';

 echo number_format($d, '2') . "<br/>";

 $e = '88.9643';

 echo number_format($e, '2') . "<br/>";

 分别输出：

 10.00

 100,000.00

 5,458.57

 1,254.90

 88.96