php保留2位小数，只舍不入

<?php
/**
* 小数保留后2位，只舍不入
*/
function round_num($num){
if($len = strpos($num,'.')){
$dian_num = substr($num,$len+1,$len+3);//获取小数点后面的数字
if(strlen($dian_num) >= 2){//判断小数点后面的数字长度是否大于2
$new_num = substr($num,0,$len+3);
}else{//补0
$new_num = $num.'0';
}
}else{//补.00
$new_num = $num.'.00';
}
return $new_num;
}
$num = 20.1111;
echo round_num($num);
?>