Rounding down the PHP 2 decimal places

Scenario : for example, platform users had a sum of money 79999.2, currency and RMB exchange ratio of 1000: 1, you need more than these "coins" into RMB deposit library

Requirements : two decimal places, rounded down

Code : 

$coins = 79999.2;  
$p = 1000;
echo  floor ( ($coins / $p) *  100 ) / 100;

The results : 79.99