public static function Apci13($date1, $date2, iauASTROM &$astrom, &$eo) {
    $ehpv = [];
    $ebpv = [];
    $r    = [];
    $x;
    $y;
    $s;

    /* Earth barycentric & heliocentric position/velocity (au, au/d). */
    IAU::Epv00($date1, $date2, $ehpv, $ebpv);

    /* Form the equinox based BPN matrix, IAU 2006/2000A. */
    IAU::Pnm06a($date1, $date2, $r);

    /* Extract CIP X,Y. */
    IAU::Bpn2xy($r, $x, $y);

    /* Obtain CIO locator s. */
    $s = IAU::S06($date1, $date2, $x, $y);

    /* Compute the star-independent astrometry parameters. */
    IAU::Apci($date1, $date2, $ebpv, $ehpv[0], $x, $y, $s, $astrom);

    /* Equation of the origins. */
    $eo = IAU::Eors($r, $s);

    /* Finished. */
  }