API:Calling_internally had bad-looking PHP examples. They were using
<source lang="php-brief"> $params = new DerivativeRequest( $this->getRequest(), array( ... </source>
The generated HTML has CSS spans in it like <div class="php-brief source-php-brief"> ... <span class="kw3">array</span>... </div>, but there are no styles for php-brief so the code isn't colored. And there's some weird framing.
The workaround is easy, just switch to lang=php.
To fix maybe lang=php-brief should request its own ext.geshi.language.php-brief ResourceLoader styles module or reuse the php CSS, but it's probably easier to just deprecate it and/or treat php-brief as a synonym for lang=php.
Meanwhile I've deprecated php-brief in https://www.mediawiki.org/wiki/Extension:SyntaxHighlight_GeSHi