This changeset introduces a `WP_Shutdown_Handler` class that detects fatal errors and which extension (plugin or theme) causes them. Such an error is then recorded, and an error message is displayed. Subsequently, in certain protected areas, for example the admin, the broken extension will be paused, ensuring that the website is still usable in the respective area. The major benefit is that this mechanism allows site owners to still log in to their website, to fix the problem by either disabling the extension or solving the bug and then resuming the extension.
Extensions are only paused in certain designated areas. The frontend for example stays unaffected, as it is impossible to know what pausing the extension would cause to be missing, so it might be preferrable to clearly see that the website is temporarily not accessible instead.
The fatal error recovery is especially important in scope of encouraging the switch to a maintained PHP version, as not necessarily every WordPress extension is compatible with all PHP versions. If problems occur now, non-technical site owners that do not have immediate access to the codebase are not locked out of their site and can at least temporarily solve the problem quickly.
Websites that have custom requirements in that regard can implement their own shutdown handler by adding a `shutdown-handler.php` drop-in that returns the handler instance to use, which must be based on a class that inherits `WP_Shutdown_Handler`. That handler will then be used in place of the default one.
Websites that would like to modify specifically the error template displayed in the frontend can add a `php-error.php` drop-in that works similarly to the existing `db-error.php` drop-in.
Props afragen, bradleyt, flixos90, ocean90, schlessera, SergeyBiryukov, spacedmonkey.
Fixes#44458.
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git-svn-id: http://core.svn.wordpress.org/trunk@44355 1a063a9b-81f0-0310-95a4-ce76da25c4cd
