<?PHP
function sterrenbeeld ($value) {

	list ($year, $month, $day) = explode("-", $value);

	if ( ($month == 1 && $day >= 20) || ($month == 2 && $day <= 18) ):

		return "Waterman";

	elseif ( ($month == 2 && $day >= 19) || ($month == 3 && $day <= 20) ):

		return "Vissen";
     
	elseif ( ($month == 3 && $day >= 21) || ($month == 4 && $day <= 19) ):

		return "Ram";
     
	elseif ( ($month == 4 && $day >= 20) || ($month == 5 && $day <= 20) ):

		return "Stier";
     
	elseif ( ($month == 5 && $day >= 21) || ($month == 6 && $day <= 21) ):

		return "Tweelingen";
     
	elseif ( ($month == 6 && $day >= 22) || ($month == 7 && $day <= 22) ):

		return "Kreeft";
     
	elseif ( ($month == 7 && $day >= 23) || ($month == 8 && $day <= 22) ):

		return "Leeuw";
     
	elseif ( ($month == 8 && $day >= 23) || ($month == 9 && $day <= 22) ):

		return "Maagd";
     
	elseif ( ($month == 9 && $day >= 23) || ($month == 10 && $day <= 22) ):

		return "Weegschaal";
     
	elseif ( ($month == 10 && $day >= 23) || ($month == 11 && $day <= 21) ):

		return "Schorpioen";
     
	elseif ( ($month == 11 && $day >= 22) || ($month == 12 && $day <= 21) ):

		return "Boogschutter";
     
	elseif ( ($month == 12 && $day >= 22) || ($month == 1 && $day <= 19) ):

		return "Steenbok";

	endif;

}
?>

Op verzoek van Crispijn een Array gebruikt.

Bron:
http://www.phphulp.nl/php/scripts/1/859/

<?PHP
function sterrenbeeld ($value) {

	list ($year, $month, $day) = explode("-", $value);

	$signs = array(
		array (19, "Steenbok"),
		array (18, "Waterman"),
		array (20, "Vissen"),
		array (19, "Ram"),
		array (20, "Stier"),
		array (21, "Tweeling"),
		array (22, "Kreeft"),
		array (22, "Leeuw"),
		array (22, "Maagd"),
		array (22, "Weegschaal"),
		array (21, "Schorpioen"),
		array (21, "Boogschutter"),
		array (19, "Steenbok")
	);

	return ($day <= $signs[$month-1][0]) ? $signs[$month-1][1] : $signs[$month][1];
}
?>