PHP : empty 결과값..(헷갈려서;)

<?
$a1 = " ";
$a = ' ';
$b = '';
$b1 = "";
$c = 0;
$d = '0';
$e = "0";
echo empty($a1)."\n";
echo empty($a)."\n";
echo empty($b)."\n";
echo empty($b1)."\n";
echo empty($c)."\n";
echo empty($d)."\n";
echo empty($e)."\n";
?>
결과 :
0
0
1
1
1
1
1