这是允许的吗？对我不能使用的东西有任何限制吗？
解决方法:
根据manual：
The key can either be an integer or a string. The value can be of any
type.Additionally the following key casts will occur:
- Strings containing valid integers will be cast to the integer type. E.g. the key “8” will actually be stored under 8. On the other hand “08” will not be cast, as it isn’t a valid decimal integer.
- Floats are also cast to integers, which means that the fractional part will be truncated. E.g. the key 8.7 will actually be stored under 8.
- Bools are cast to integers, too, i.e. the key true will actually be stored under 1 and the key false under 0.
- Null will be cast to the empty string, i.e. the key null will actually be stored under “”.
- Arrays and objects can not be used as keys. Doing so will result in a warning: Illegal offset type.
手册again：
A string is series of characters, where a character is the same as a byte. This means that PHP only supports a 256-character set, and hence does not offer native Unicode support. See details of the string type.
简而言之,任何字符串都可以成为关键.字符串可以包含任何二进制数据(最多2GB).因此,密钥可以是任何二进制数据(因为字符串可以是任何二进制数据).
一些随机(有效)滥用数组键：
$w = array(chr(0) => 'null byte?', chr(rand(0, 255)) => 'random byte?');
var_dump($w);
