WP_Ajax_Upgrader_Skin Bug: "unique" error code is always 1

[jrf]

The concatenation and plus/minus signs in PHP have the ​same operator precedence.

With that in mind, have a look at the following code in wp-admin/includes/class-wp-ajax-upgrader-skin.php:

<?php
// Count existing errors to generate an unique error code.
$errors_count = count( $this->errors->get_error_codes() );
$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1, $string );

The "unique" error code, will basically always be 1 as the concatenation is done first, the resulting string which doesn't start with a number is then cast to an integer - 0 - before the addition of 1 is done and 0 + 1 will always be 1.

See: ​https://3v4l.org/EYuU8

While this is a bug in its own right as is, as of PHP 8.0, the operator precedence of concatenation will be lowered, with deprecation notices being thrown as of PHP 7.4 for unparenthesized expressions containing an '.' before a '+' or '-'.

Also see:

• ​https://wiki.php.net/rfc/concatenation_precedence

[jrf]

Note: this is the only code within WordPress which is affected by this PHP 7.4/8.0 change based on an analysis done by Nikita Popov of PHP core before this change was implemented.
See:

• ​http://news.php.net/php.internals/105442
• ​https://gist.github.com/nikic/a4df3e8e18c7955c2c21cf6cdb4cbfaa

FYI: there will soon be a sniff available in ​PHPCompatibility to detect code affected by this change so userland code/plugins/themes can verify whether they are affected.

[SergeyBiryukov]

In 45487:

Upgrade/Install: Correct operator precedence for an error code in WP_Ajax_Upgrader_Skin::error().

Props jrf.
Fixes #47441.

[desrosj]

Dev note: ​https://make.wordpress.org/core/2019/10/11/wordpress-and-php-7-4/