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geshi highligher is a resource consumer and it could slow down your page loading thus geshi is not enabled by default.
To enable geshi, modify /class/textsanitizer/syntaxhighlight/config.php
return $config = array(
'highlight' => 'geshi', // Source code highlight: '' - disable; 'php' - php highlight; 'geshi' - geshi highlight
'language' => 'PHP', // Default language for code highlight, applicable only if geshi is enabled
);