Remove redundant polyfills for PHP native functionality (excl JSON)

[jrf]

Now the new minimum PHP requirement for WordPress is PHP 5.6.20, a number of the polyfills WordPress provides for PHP functionality can be removed.

I will be attaching the relevant patches for this to this ticket.

Let's talk through them:

array_replace_recursive()

The ​`array_replace_recursive()` function was introduced in PHP 5.3.0.

The function has had no significant changes since then which would need to be accounted for.

The Array extension is part of ​PHP Core and cannot be disabled.

Conclusion: this polyfill can now be safely removed.

SPL autoload

WP polyfills the following PHP native SPL extension functionality:

• The function ​`spl_autoload_register()` (PHP 5.1.0+)
• The function ​`spl_autoload_unregister(()` (PHP 5.1.0+)
• The function ​`spl_autoload_functions()` (PHP 5.1.0+)

These function have had no significant changes since then which would need to be accounted for.

​Prior to PHP 5.3.0, the SPL extension could be disabled via a compilation flag - and sometimes was -. As of PHP 5.3.0, the extension can no longer be disabled.

Conclusion: these polyfills can now be safely removed.

Implementation

This polyfill uses a separate file wp-includes/spl-autoload-compat.php.
While I suspect this to be exceedingly rare in practice, plugins/themes could load this file directly.

So, there is a choice to make here:

• Remove the file completely.
• Remove the code and deprecate the file.

In line with WP's general cautiousness in these cases, I have opted for the second choice and have removed the code from the file, replacing it with a call to _deprecated_file() to alert any plugins/themes which would still try to include the file directly.

Other considerations

The removed code includes a WP specific global variable $_wp_spl_autoloaders which was conditionally declared when the polyfill was active.
This global variable will no longer be available.

While I don't expect any plugins/themes access this variable directly, removing this variable could be considered a BC-break.
All the same, as the variable would only be available on PHP < 5.3 when the SPL extension was disabled, one can expect that any userland code which would access this variable directly, would be accompanied by a call to isset(), so removing this variable should - in practice - not cause any problems.

Hash

WP polyfills the following PHP native Hash extension functionality:

• The function ​`hash_hmac()` (PHP 5.1.2+)
• The function ​`hash_equals()` (PHP 5.6.0+)

These functions have had no significant changes since then which would need to be accounted for.

While one may think those can now be safely removed, this is not the case.

While the Hash extension has been bundled and compiled with PHP core since PHP 5.1.2, until PHP 7.4, the Hash extension could still be ​disabled at compile time using the `--disable-hash` flag.

Conclusion: Based on the above, these two functions should not be removed at this time.

Implementation

I have added some additional documentation to these functions, making it explicit when will be the right time to remove them.

Unit tests

As these functions are used throughout core, removing them can be considered sufficiently unit tested when all the WP Core unit tests pass, and they still do: ​https://travis-ci.com/WordPress/wordpress-develop/builds/119063509

Other

• Ticket https://core.trac.wordpress.org/ticket/46630 is a partial duplicate of this ticket.
• The JSON functionality related polyfills will be addressed in a separate ticket, which I will link to once I've created it.

/cc @pento

[jrf]

Ticket for the JSON polyfill removal: #47699

[pento]

#46630 was marked as a duplicate.

[pento]

In 45636:

Code Modernisation: Remove the array_replace_recursive() polyfill.

This function was added in PHP 5.3.0, so we no longer need the polyfill.

Props jrf.
See #47698.

[pento]

In 45637:

Code Modernisation: Remove the SPL autoloader polyfill.

As of PHP 5.3, the SPL extension cannot be disabled, so we no longer need this polyfill.

The file is kept with a _deprecated_file() call, to alert any plugins or themes that may be loading it directly.

Props jrf, ayeshrajans.
See #47698, #46630.

[pento]

In 45638:

Code Modernisation: Document when the Hash polyfills can be removed.

The Hash extension cannot be disabled as of PHP 7.4. So, while we can't remove these polyfills yet, we can document when we'll be able to.

Props jrf.
Fixes #47698.