PHP删除变量unset()和null

2. $a = array(
3. 'a' => 'a',
4. 'b' => 'b'
5. );
6. $b = array(
7. 'a' => 'a',
8. 'b' => 'b'
9. );
10. $a['b'] = null;
11. unset($b['b']);
12. print('

    ');
    print_r($a);
    print('
    ');
    print_r($b);
    print('

    ');
13. ?>

结果： Array ( [a] => a => ) Array ( [a] => a )