PHPのarray_key_existsとissetの違いと注意点

<?php
function check($array, $name) {
        $r = array_key_exists($name, $array);
        $i = isset($array[$name]);

        echo "$name:",PHP_EOL;
        echo "array_key_exists = ";
        var_dump($r);
        echo "isset = ";
        var_dump($i);
        echo PHP_EOL;
}

$a = array ('null'=>null, 'false' => false, 'hoge'=>'meta');

check($a, 'null');
check($a, 'false');
check($a, 'hoge');
?>

% php array_test.php
null:
array_key_exists = bool(true)
isset = bool(false)

false:
array_key_exists = bool(true)
isset = bool(true)

hoge:
array_key_exists = bool(true)
isset = bool(true)