This function is similar to
domdocument_get_elements_by_tagname() but searches for
an element with a given id. According to the DOM standard this requires a
DTD which defines the attribute ID to be of type ID, though the current
implementation simply does an xpath search for "//*[@ID = '%s']". This
does not comply to the DOM standard which requires to return null if it
is not known which attribute is of type id. This behaviour is likely to
be fixed, so do not rely on the current behaviour.