lib/SteamCondenser/Community/SteamId.php
public static function convertCommunityIdToSteamId($communityId) {
        $steamId1 = bcmod($communityId, 2);
        $steamId2 = bcsub($communityId, 76561197960265728);

        if ($steamId2 <= 0) {
            throw new SteamCondenserException("SteamID $communityId is too small.");
        }

        $steamId2 = ($steamId2 - $steamId1) / 2;

        return "STEAM_0:$steamId1:$steamId2";
    }
public static function convertCommunityIdToSteamId($communityId) {
        $steamId1 = bcmod($communityId, 2);
        $steamId2 = bcsub($communityId, 76561197960265728);

        if ($steamId2 <= 0) {
            throw new SteamCondenserException("SteamID $communityId is too small.");
        }

        $steamId2 = ($steamId2 - $steamId1) / 2;

        return "STEAM_0:$steamId1:$steamId2";
    }
Converts a 64bit numeric SteamID as used by the Steam Community to a
SteamID as reported by game servers

@param string $communityId The SteamID string as used by the Steam
Community
@return string The converted SteamID, like <var>STEAM_0:0:12345</var>
@throws SteamCondenserException if the community ID is to small