（二元）铸造实际上做了什么以及为什么不应该依赖它？

Question

I'm using PHP 7.2.12

I come across following statement from the Type Casting section of PHP Manual :

(binary) casting and b prefix forward support was added in PHP 5.2.1. Note that the (binary) cast is essential the same as (string), but it should not be relied upon.

I didn't understand the above text thoroughly. Someone please explain to me with good explanation.

I studied the following code examples given in PHP Manual on the same page :

<?php
$binary = (binary) $string;
var_dump($binary);
$binary = b"binary string";
var_dump($binary);
?>

Output :

Notice: Undefined variable: string in ..... on line 2
string(0) ""
string(13) "binary string"

If you look at the output above I got the same strings even after the casting to binary. So, what conversion job does binary casting actually do?

Why the binary casting should not be relied upon?

Also, explain to me on what types the binary casting can be done? I mean it's legal.

Nowhere in the PHP manual, there is any explanation or justification in this regard.

Someone please help me out on this by guiding me in a right direction.

Answer 1

PHP had Big Plans™ for PHP 6, where strings would finally become Unicode strings. To illustrate what that means, the current PHP behaviour:

$str = '漢字';
echo $str[0];
// ?

In PHP 6, this would have output "漢" instead of a broken ?. In other words, strings are encoding and character aware, instead of dumb byte arrays. (To output "漢" in current PHP versions, you need something like mb_substr($str, 0, 1, 'UTF-8').)

To keep the old dumb-byte-array behaviour, you could prefix your strings with b'漢字' and you could cast Unicode strings to dumb byte arrays using (binary). This was all added to PHP 5 in preparation for PHP 6, so you could start updating your code in advance.

Well, except PHP 6 never happened, and b'' prefixes and (binary) casts still don't do anything to this date.