TYPO3 Core

The parameter $info is not a string but an array:

/**
 * Interface for classes which hook into PageLayoutView and do additional
 * tt_content_drawFooter processing.
 */
interface PageLayoutViewDrawFooterHookInterface
{
    /**
     * Preprocesses the preview footer rendering of a content element.
     *
     * @param \TYPO3\CMS\Backend\View\PageLayoutView $parentObject Calling parent object
     * @param string $info Processed values
     * @param array $row Record row of tt_content
     */
    public function preProcess(\TYPO3\CMS\Backend\View\PageLayoutView &$parentObject, &$info, array &$row);

See function "tt_content_drawFooter", if you want an example of $info and how function "preProcess" is called.