lib/SteamCondenser/Community/SteamId.php
public static function convertSteamIdToCommunityId($steamId) {
        if($steamId == 'STEAM_ID_LAN' || $steamId == 'BOT') {
            throw new SteamCondenserException("Cannot convert SteamID \"$steamId\" to a community ID.");
        }
        if (preg_match('/^STEAM_[0-1]:[0-1]:[0-9]+$/', $steamId)) {
            $steamId = explode(':', substr($steamId, 8));
            return bcadd($steamId[0] + $steamId[1] * 2, 76561197960265728);
        } elseif (preg_match('/^\[U:[0-1]:[0-9]+\]$/', $steamId)) {
            $steamId = explode(':', substr($steamId, 3, strlen($steamId) - 1));
            return bcadd($steamId[0] + $steamId[1], 76561197960265727);
        } else {
            throw new SteamCondenserException("SteamID \"$steamId\" doesn't have the correct format.");
        }
    }
public static function convertSteamIdToCommunityId($steamId) {
        if($steamId == 'STEAM_ID_LAN' || $steamId == 'BOT') {
            throw new SteamCondenserException("Cannot convert SteamID \"$steamId\" to a community ID.");
        }
        if (preg_match('/^STEAM_[0-1]:[0-1]:[0-9]+$/', $steamId)) {
            $steamId = explode(':', substr($steamId, 8));
            return bcadd($steamId[0] + $steamId[1] * 2, 76561197960265728);
        } elseif (preg_match('/^\[U:[0-1]:[0-9]+\]$/', $steamId)) {
            $steamId = explode(':', substr($steamId, 3, strlen($steamId) - 1));
            return bcadd($steamId[0] + $steamId[1], 76561197960265727);
        } else {
            throw new SteamCondenserException("SteamID \"$steamId\" doesn't have the correct format.");
        }
    }
Converts a SteamID as reported by game servers to a 64bit numeric
SteamID as used by the Steam Community

@param string $steamId The SteamID string as used on servers, like
<var>STEAM_0:0:12345</var>
@return string The converted 64bit numeric SteamID
@throws SteamCondenserException if the SteamID doesn't have the correct
format