$a = '';
echo '1.-----------'.($a == '').'<br>';
echo '2.-----------'.($a == null).'<br>';
echo '3.-----------'.($a == false).'<br>';
echo '4.-----------'.($a == '0').'<br>';
echo '5.-----------'.($a == 0).'<br>';
$a = null;
echo '6.-----------'.($a == '').'<br>';
echo '7.-----------'.($a == null).'<br>';
echo '8.-----------'.($a == false).'<br>';
echo '9.-----------'.($a == '0').'<br>';
echo '10.-----------'.($a == 0).'<br>';
$a = '0';
echo '11.-----------'.($a == '').'<br>';
echo '12.-----------'.($a == null).'<br>';
echo '13.-----------'.($a == false).'<br>';
echo '14.-----------'.($a == '0').'<br>';
echo '15.-----------'.($a == 0).'<br>';
$a = 0;
echo '16.-----------'.($a == '').'<br>';
echo '17.-----------'.($a == null).'<br>';
echo '18.-----------'.($a == false).'<br>';
echo '19.-----------'.($a == '0').'<br>';
echo '20.-----------'.($a == 0).'<br>';
$a = false;
echo '21.-----------'.($a == '').'<br>';
echo '22.-----------'.($a == null).'<br>';
echo '23.-----------'.($a == false).'<br>';
echo '24.-----------'.($a == '0').'<br>';
echo '25.-----------'.($a == 0).'<br>';
输出的结果
1.-----------1
2.-----------1
3.-----------1
4.-----------
5.-----------1
6.-----------1
7.-----------1
8.-----------1
9.-----------
10.-----------1
11.-----------
12.-----------
13.-----------1
14.-----------1
15.-----------1
16.-----------1
17.-----------1
18.-----------1
19.-----------1
20.-----------1
21.-----------1
22.-----------1
23.-----------1
24.-----------1
25.-----------1

$a = '';//0|'0'|false|null 输出的结果一致
echo '1.-------'.(!empty($a) == '').'<br>';
echo '2.-------'.(!empty($a) == null).'<br>';
echo '3.-------'.(!empty($a) == false).'<br>';
echo '4.-------'.(!empty($a) == '0').'<br>';
echo '5.-------'.(!empty($a) == 0).'<br>';

输出的结果：
1.-------1
2.-------1
3.-------1
4.-------1
5.-------1