Steps to replicate the issue (include links if applicable):

• Enable WSOAuth
• Enter to Special:Preferences

What happens?: White screen of death.

What should have happened instead?: Display the preferences page.

Software version: MediaWiki 1.40

Other information (browser name/version, screenshots, etc.): In the apache2 error log it states:

PHP Fatal error:  Declaration of MWCallbackStream::write($string) must be compatible with Psr\Http\Message\StreamInterface::write(string $string): int in /var/www/wikisp/wiki/includes/http/MWCallbackStream.php on line 49.

The MediaWiki core was downloaded via the download link at MediaWiki.org (no git was used). Initially WSOAuth was downloaded via git (used composer install --no-dev on the extension folder) and then the MW 1.39 and 1.40 compatible versions respectively via the extension distributor to check whether it was due to being out of date. Momentarily uninstalled the extension on the wikispwiki.