In PHP 5.4, the new method signature (where we now pass $this) is incompatible with the parent class, causing a notice to be emitted when E_STRICT is set.
The quick fix is to roll-back this aspect of .
(In ) fixes #2806, refs #2809 - quick fix
(In ) fixes #2809, refs #2714: making signature of Piwik_DataTable_Renderer::renderHeader() compatible across subclasses
(In ) refs #2809 - refactoring