public function phi1z($eccent, $qs)
    {
        $phi = Common::asinz(0.5 * $qs);

        if ($eccent < Common::EPSLN) {
            return $phi;
        }

        $eccnts = $eccent * $eccent;

        for ($i = 1; $i <= 25; ++$i) {
            $sinphi = sin($phi);
            $cosphi = cos($phi);
            $con = $eccent * $sinphi;
            $com = 1.0 - $con * $con;
            $dphi = 0.5 * $com * $com / $cosphi * ($qs / (1.0 - $eccnts) - $sinphi / $com + 0.5 / $eccent * log((1.0 - $con) / (1.0 + $con)));
            $phi = $phi + $dphi;

            if (abs($dphi) <= 1e-7) {
                return $phi;
            }
        }

        Proj4php::reportError("aea:phi1z:Convergence error");

        return null;
    }