This has been done to match PHP's syntax for user defined
functions more closely[1].

[1] <https://github.com/php/phd/pull/37Y>

Actually, this case is not related to the mentioned PR, but rather
because variable-length argument lists[1] can be empty.

[1] <https://www.php.net/manual/en/functions.arguments.php#functions.variable-arg-list>

Let's look at crypt function:

crypt ( string $string , string $salt ) : string # current documentation

and look at previous documentation:

crypt ( string $str [, string $salt ] ) : string # in this case we know that second argument is optional. In current version we don't know.

This argument is no longer optional as of PHP 8.0.0[1].

[1] <https://www.php.net/manual/en/function.crypt.php#refsect1-function.crypt-changelog>

I understand. I'm preparing for Zend Certified PHP Engineer exam, which bases on PHP 7.1. It is possible to read or download old documentation for this version? Thank You very much in advance.

There is no readily downloadable or viewable documentation for PHP
7 only.  However, you can checkout an older revision of the doc
repo[1], and build the documentation yourself[2].

[1] <https://github.com/php/doc-en/commit/3fc4586f54eb27f406b848058941217493110f83>
[2] <https://github.com/php/doc-base/blob/master/README.md#quick-reference>

I can't rebuild documentation for PHP 7.1. I haven't enough knowledge to do this. This documentation was available as tar.gz, one html, many html, zip files to download. I thought, that there are some archive with older documentation. Thank You very much for help, Your assistance.

Then consider to use the documentation available at Zend[1],
or download the archived docs[2].  While both are named PHP 5
documentation, they already contain a lot of documentation
about PHP 7.

[1] <https://php-legacy-docs.zend.com/manual/php5/en/index>
[2] <http://doc.php.net/archives/>