Replace certain `array_key_exists()` calls with `isset()` once PHP requirement is raised to 7.0+

[flixos90]

This ticket is blocked until WordPress requires at least PHP 7.0 as minimum version.

Checks like isset( MY_CONSTANT[ $key ] ) result in a fatal error in PHP < 7.0, which is why parts of the WordPress codebase uses array_key_exists() for such situations as of today, due to the support for PHP 5.6. Also see ​https://github.com/PHPCompatibility/PHPCompatibility/issues/1301 which explores flagging this as part of the PHPCompatibility sniffs.

This was raised as part of #56974, which reviewed several of these checks. isset is slightly preferable over array_key_exists in terms of performance (with the caveat that the two behave differently when it comes to null values).

As of today, this ticket is essentially a reminder to update such calls to array_key_exists() in WordPress core once it will at some point no longer support PHP versions below 7.0.

[flixos90]

cc @jrf @aristath @desrosj

[flixos90]

In 54804:

Editor: Improve performance of WP_Theme_JSON class by reducing usage of expensive array functions.

In many scenarios array functions are more expensive than using simpler for or foreach loops.

This changeset results in roughly 4% faster wp_head execution time for both block themes and classic themes. While this may seem like a small win, it is a worthwhile enhancement and only one part of several other little performance tweaks which are being worked on to improve performance of theme.json parsing further.

Props aristath, desrosj, jrf, spacedmonkey.
Fixes #56974.
See #57067.

[flixos90]

In 54805:

Editor: Improve performance of WP_Theme_JSON class by reducing usage of expensive array functions.

In many scenarios array functions are more expensive than using simpler for or foreach loops.

This changeset results in roughly 4% faster wp_head execution time for both block themes and classic themes. While this may seem like a small win, it is a worthwhile enhancement and only one part of several other little performance tweaks which are being worked on to improve performance of theme.json parsing further.

Props aristath, desrosj, jrf, spacedmonkey.
Merges [54804] to the 6.1 branch.
Fixes #56974.
See #57067.

PR fixes a small regression after 54805, where an incorrect variable was used inside the loop.

Trac ticket: https://core.trac.wordpress.org/ticket/57067

[flixos90]

In 55142:

Editor: Fix undefined variable following [54805].

Props mamaduka, costdev, mukesh27.
See #56974, #57067.

Committed in https://core.trac.wordpress.org/changeset/55142.

[joemcgill]

The effort to bump the minimum version of PHP is being tracked in #57345, which is blocking this ticket. Even so, it would be good to have a patch ready and reviewed so it can be committed as soon as the official decision to drop PHP 5.6 has been made.