From Organic Design

To use ckeditor with mediawiki:

download and unpack the source ckeditor package place it in the extensions directory of mediawiki add the following lines to the end of mediawiki LocalSettings.php:

require_once( "$IP/extensions/ckeditor/ckeditor.php" );

$wgHooks['EditPage::showEditForm:initial'][] = 'showEditForm';
function showEditForm( $form ) {
    global $wgOut;
    $wgOut->addScriptFile( '/extensions/ckeditor/ckeditor.js' );
    $script = "<script type=\"text/javascript\">
        window.onload = function() { CKEDITOR.replace( 'wpTextbox1' ); };
        if( window.removeEventListener ) window.removeEventListener( 'load', mwSetupToolbar, false );
        else if( window.detachEvent ) window.detachEvent( 'onload', mwSetupToolbar ) ;
    $wgOut->addHTML( $script );
    return true;

This is from mwforums.