lang/php81: add PHP 8.1.0 release

PHP is a widely-used open source general-purpose scripting language
that is especially suited for web development and can be embedded
into HTML.  It is modular, and object-oriented.  Much of its syntax
is borrowed from C, Java and Perl with a couple of unique PHP-specific
features thrown in.  The language is designed to allow web developers
to write dynamically generated pages quickly.

PHP 8.1 comes with numerous improvements and new features such as

* Enumerations
* Readonly properties
* Fibers
* Pure Intersection Types
* never return type
* First-class Callable Syntax
* "final" modifier for class constants
* New fsync and fdatasync functions
* New array_is_list function
* Explicit Octal numeral notation
* And much much more...

(taca)