- floor: documentation ( source)
- pow: documentation ( source)
<?php
$current_skill = 7000;
$mod = (- 0.0087 * pow(floor($current_skill/1000),3)) + 0.0346 * pow(floor($current_skill/1000),2) - 0.1042 * pow(floor($current_skill/1000),1) + 3.0061;
echo $mod;