$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