Characters allowed in php array keys?

According to the 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.

The manual 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.

So in short, any string can be a key. And a string can contain any binary data (up to 2GB). Therefore, a key can be any binary data (since a string can be any binary data).

Some random (valid) abuse of array keys:

$w = array(chr(0) => 'null byte?', chr(rand(0, 255)) => 'random byte?');var_dump($w);

The key must be a string or an integer. There are some casts that take place, but I think the manual does a good job of explaining:

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.

I found this answer looking for more information on a problem I had. I was using strings with UTF-8 characters in them, which would not work as keys to an array I had.

Something like

$str = "R&D - Solution";$arr = array( "R&D - Solution" => "Research" );echo $arr[$str];  // did not work

The (not big or clever) solution for me was to do this..

$str = md5("R&D - Solution");$arr = array( md5("R&D - Solution") => "Research" );echo $arr[$str];  // works!