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.
See also DomDocument_get_elements_by_tagname()