The DonateBoxInSidebar extension allows an automatic addition of a PayPal donate button or box to a wikis sidebar.
Download and upload the zip file to /extensions and extract. Rename directory folder to DonateBoxInSidebar and add the following to LocalSettings.php to enable this extension:
wfLoadExtension( 'DonateBoxInSidebar' );
See https://www.mediawiki.org/wiki/Extension:DonateBoxInSidebar for more information about this extension.