Inverser une function

function lvl($level) {
		$whole = floor($level);
		$partial = $level - $whole;

		$odd = ceil($whole/2);
		$even = floor($whole/2);
		$oddXp = ($odd * ($odd + 1) / 2) * 7;
		$evenXp = ($even * ($even + 1) / 2) * 7 + $even * 3;
		$wholeXp = $oddXp + $evenXp;
		$partialXp = (7 + floor(($whole + 1) * 7 / 2)) * $partial;

		return ($wholeXp + $partialXp);
	}