[PHP-DEV] Warn when declaring required parameter after optional one

[Nikita Popov]

Hi internals,

I've created https://github.com/php/php-src/pull/5067 to make code like

    function test($foo = null, $bar) {}

throw a warning:

    Warning: Required parameter $bar follows optional parameter

Historically, having an "optional" parameter before a required one was
useful for poor man's nullable types. That is, one could write

    function test(FooBar $param = null, $param2)

to get an effective

    function test(?FooBar $param, $param2)

signature on old PHP versions that did not have native support for nullable
types.

Since nullable types have been available since PHP 7.1, having a required
parameter after an optional one is increasingly likely a bug rather than an
intentional workaround, so I think it would be good to throw a warning for
this case.

What do you think?

Nikita