PHP function code to determine constellation based on date

function yige_constellation($month, $day) {
 // Check parameter validity
 if ($month < 1 || $month > 12 || $ day < 1 || $day > 31) return false;
 // Constellation name and start date
 $constellations = array(
 array( "20" => "Aquarius" ),
 array( "19" => "Pisces"),
 array( "21" => "Aries"),
 array( "20" => "Taurus"),
 array( "21" => "Gemini"),
 array( "22" => "Cancer"),
 array( "23" => "Leo"),
 array( "23" => "Virgo") ,
 array( "23" => "Libra"),
 array( "24" => "Scorpio"),
 array( "22" => "Sagittarius"),
 array( "22" => "Capricorn")
 );
 list($constellation_start, $constellation_name) = each($constellations[(int)$month-1]);
  if ($day < $constellation_start) list($constellation_start, $constellation_name) = each($constellations[($month -2 < 0) ? $month = 11: $month -= 2]);
 return $constellation_name;
}
//Call:
echo yige_constellation(4, 20);