Description:
------------
First : my PHP version is 5.6.19, but it's not listed above.
When using DOMNode::replaceChild or DOMNode::appendChild or DOMNode::insertBefore in order to insert a node which includes long namespace prefixes (ex. : "variable_trade_item_information:variableTradeItemInformationModule"), the prefix is truncate to 20 characters. In this example it becomes "variable_trade_item_:variableTradeItemInformationModule".

When inserting such a node, DOM also adds in the root tag of the node all the definitions of the prefix used in the node. This bunch of prefix definitions  include the same truncated prefixes.
May be this header is created because the truncated prefixes are not declared (after they have been truncated), as their definitions in the tag itself are not truncated. Ex; : <variable_trade_item_:variableTradeItemInformationModule xmlns:variable_trade_item_information="urn:gs1:gdsn:variable_trade_item_information:xsd:3" ... (snipped) >

Test script:
---------------
Sorry but my test script depends on large XML file...