php数组怪事，求解答

class dd {

    const d = 'd';

    public function test() {

        $arr = array();
        $arr['d'] = 'test';

        $arr2 = array(
                'a' => '1',
                'b' => '2',
                'c' => '3',
                'd' => '4'
            );

        foreach ($arr2 as $key=>$value) {
            if (in_array($key,array('a','b','c')) || isset($arr['self::d'])) {
                echo $key;
            }
        }
    }
}

$dd = new dd();
$dd->test();

输出：

abc

为什么不是输出abcd

========补充一下======== 其实我真正想问的是，单引号不是不解析么？为什么echo $arr['self::d']。提示的是未定义'self::d'这个常量？提示是为定义'self::d'可不是'd'哦。

=======我是分割线========

回复内容：

class dd {

    const d = 'd';

    public function test() {

        $arr = array();
        $arr['d'] = 'test';

        $arr2 = array(
                'a' => '1',
                'b' => '2',
                'c' => '3',
                'd' => '4'
            );

        foreach ($arr2 as $key=>$value) {
            if (in_array($key,array('a','b','c')) || isset($arr['self::d'])) {
                echo $key;
            }
        }
    }
}

$dd = new dd();
$dd->test();

输出：

abc

为什么不是输出abcd

========补充一下======== 其实我真正想问的是，单引号不是不解析么？为什么echo $arr['self::d']。提示的是未定义'self::d'这个常量？提示是为定义'self::d'可不是'd'哦。

=======我是分割线========

虽然已经有了不少回答了我还是说一下我的看法 ...

关于为什么不是输出 abcd 楼上说了很多我就不赘述了 ... 我单单说一下你遇到的这个错误 ...

事实上 ... 这是一个 非常诡异 的错误 ... 在我的知识范围内这是不可能出现的 ...

首先 ... 一个类常量在任何情况下都不会被自动转化为字符串 ...

其次 ... 调用不存在的类常量只会引发 Warning 或者 Fatal ... 任何情况下都不会只是一个 Notice ...

最直白的证明 ... 我写了一段测试代码 ... 在兼容箱里跑了一下 ... 代码和运行报告如下 ...

bar();

------------------------------------------------------------------------

php version 5.5.0beta4
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0beta3
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0beta2
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0beta1
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0alpha6
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0alpha5
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0alpha4
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0alpha3
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0alpha2
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.5.0alpha1
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.14
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.12
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.11
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.10
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.9
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.8
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.7
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.6
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.5
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.4
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.3
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.2
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.1
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.4.0
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.24
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.22
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.21
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.20
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.19
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.18
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.17
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.16
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.15
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.14
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.13
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.12
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.11
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.10
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.9
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.8
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.7
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.6
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.5
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.4
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.3
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.2
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.1
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.3.0
    Notice: Undefined index: self::xx in cevin on line 5
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.17
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.16
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.15
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.14
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.13
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.12
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.11
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.10
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.9
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.8
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.7
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.6
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.5
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.4
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.3
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.2
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.1
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.2.0
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.6
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.5
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.4
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.3
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.2
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.1
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.1.0
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.0.5
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.0.4
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.0.3
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.0.2
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.0.1
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 5.0.0
    Notice: Undefined index: self::xx in cevin on line 5 
    Fatal error: Undefined class constant 'xx' in cevin on line 6

php version 4.4.9
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.8
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.7
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.6
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.5
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.4
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.3
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.2
    Parse error: syntax error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.1
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.4.0
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.11
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.10
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.9
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.8
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.7
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.6
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.5
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.4
    Parse error: parse error, expecting `'('' in cevin on line 6

php version 4.3.3
    Parse error: parse error, expecting `'('' in cevin on line 6

php version 4.3.2
    Parse error: parse error, expecting `'('' in cevin on line 6

php version 4.3.1
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

php version 4.3.0
    Parse error: parse error, unexpected ']', expecting '(' in cevin on line 6

狮屎胜于熊便 ... 通过这份报告基本可以断定这是你的 php 环境的问题 ...

不用怀疑自己的知识 ... 在这件事情上你对单引号的理解是对的 ... 不要被误导了 ...

以及说 ... 如果你用的是 AppServ 提供的那个 php 6.0.0-dev 版本的 php 的话 ... 尽早换了吧 ...

恩 ... 就是这样啦 ...

你的代码写错了 $arr['self::d'] 应该修改为 $arr[self::d]

朋友关键看这里哦

in_array($key,array('a','b','c'))

这说明$key范围只能是在a,b,c这三个范围才能输出$key

再说说判断条件中的isset($arr['self::d'])

在php中双引号中的变量是可以执行的，而单引号是不解析变量的

所以$arr['self::d']是不存在的 如果你需要将self::d当作键的话需可以这么做$arr[self::d]

所有最终你得到的结果是abc

一般php在用变量做键获取数组值时，不需要加引号，如$arr[$key]
对于你给出的问题，我刚做了实验
将 isset($arr['self::d'])换成isset($arr[self::d])即可

补充

针对你的补充的notice信息，我也做了实验，echo $arr['self::d']提示信息如下：
Notice: Undefined index: self::d in D:\servers\phpweb\guai.php on line 17 abc
并没有发现和你相同的报错信息，针对这个不同，你需要确认两点：
1、是否是由于你给出的例子和你真实想要写的代码之间有哪些不同导致了这个问题。
2、你的php版本是否太低。（我用的版本是PHP Version 5.4.5）

这个问题需要先解决了你的补充问题。

1.执行 echo $arr['self::d'] 给出的notice的是：

Notice: Undefined index: self::d

注意如果你把这翻译成中文应该是未定义的索引，而非未定义的常量。单引号不解析这句话是对的，因此php会直接在数组 $arr 中寻找索引（或者说下标，看你的习惯了）为 self::d 的元素，然后未找到，因此给出notice

2.为什么输出的是 abc 而非abcd？看完了上面的一个回答者问题就很容易了，因为单引号不解析，而 $arr 中索引为 self::d 的元素不存在，因此 isset($arr['self::d']) 这一个表达式是为 false 的。修正的方法（我觉得也是你的本意）其他几位已经回答了，把 $arr['self::d'] 改为 $arr[self::d] 。没有了单引号，那 self::d 就会被解析执行，判断的就是 $arr['d'] 了。

其实只要在你调用之前用var_dump来输出一下变量的值，看看输出结果是什么就可以了。'self::d'，"self::d"始终获得的是本身的字符串。