Steps to reproduce:

1. Set up local Phabricator instance with WMF customizations on a local Debian 11 Bullseye machine with PHP 2:7.4+76 (in contrast to our current Debian 10 Phabricator production instance with an older PHP version)
2. Go to http://phabricator.lan/auth/config/new
3. Select MediaWiki - Configure MediaWiki OAuth. to go to http://phabricator.lan/auth/config/edit?provider=PhabricatorMediaWikiAuthProvider

Expected outcome:
Page "Add Auth Provider: MediaWiki" is rendered correctly.

Actual outcome:
Unhandled Exception: Error while loading file "/var/www/html/wmfphab202305/deployment/libext/misc/src/oauth/PhutilMediaWikiAuthAdapter.php": Array and string offset access syntax with curly braces is deprecated due to https://wiki.php.net/rfc/deprecate_curly_braces_array_access