Set the namespace for an XML tag with PHP

I'd like to create an XML document with a very specific format. It should look similar to this:

<?xml version="1.0" encoding="UTF-8"?>
<ram:FLOW xmlns:ram=\"http://MY_LIBRARY\" xmlns:mar=\"http://ANOTHER_LIBRARY\" xmlns:xsi=\"http://ift.tt/ra1lAU\">
<Header>
<Source>Application1</Source>
<Time>2014-11-12T12:46:39</Time>
<Environment>TEST</Environment>
<Sequence>537</Sequence>
</Header>
<Data>
<mar:OC_DC>
<DC_elements>
<Unit>
<Unit_ID>089789</Unit_ID>
<State>active</State>
</Unit>
<Unit>
<Unit_ID>459008</Unit_ID>
<State>inactive</State>
</Unit>
</DC_elements>
</mar:OC_DC>
</Data>
</ram:FLOW>

I wrote a PHP/MySQL script to generate this document:

<?php   
$xml = new SimpleXMLElement("<?xml version=\"1.0\" encoding=\"UTF-8\"?><ram:FLOW xmlns:ram=\"http://MY_LIBRARY\" xmlns:mar=\"http://ANOTHER_LIBRARY\" xmlns:xsi=\"http://ift.tt/ra1lAU\"></ram:FLOW>");

$header = $xml->addChild('Header');
$header->addChild('Source', $source);
$header->addChild('Time', $time);
$header->addChild('Environment', $env);
$header->addChild('Sequence', $sequence);

$data=$xml->addChild('Data');
$mar_oc_dc=$data->addChild('mar:OC_DC');
$dc_elements=$mar_oc_dc->addChild('DC_elements');

while($condition)
{
   // some MySQL code here to extract unit_id and state
   $unit=$dc_elements->addChild('Unit');
   $unit_id=$unit->addChild('Unit_ID', $unit_id);
   $state=$unit->addChild('State', $state);
}

$dom = new DOMDocument();
$dom->preserveWhiteSpace = FALSE;
$dom->formatOutput = TRUE;
$dom->loadXML($xml->asXML());
$handle = fopen("backup/" . $file_name . ".xml", "w");
fwrite($handle, $dom->saveXML());
fclose($handle);            
?>

But the result was a little bit different from what I expected:

<?xml version="1.0" encoding="UTF-8"?>
<FLOW xmlns:ram=\"http://MY_LIBRARY\" xmlns:mar=\"http://ANOTHER_LIBRARY\"     xmlns:xsi=\"http://ift.tt/ra1lAU\">
<Header>
<Source>Application1</Source>
<Time>2014-11-12T12:46:39</Time>
<Environment>TEST</Environment>
<Sequence>537</Sequence>
</Header>
<Data>
<OC_DC>
<DC_elements>
<Unit>
<Unit_ID>089789</Unit_ID>
<State>active</State>
</Unit>
<Unit>
<Unit_ID>459008</Unit_ID>
<State>inactive</State>
</Unit>
</DC_elements>
</OC_DC>
</Data>
</FLOW>

As you can see, the ram:FLOW tag became FLOW, and the mar:OC_DC tag became OC_DC. I looked on Stack Overflow and other websites for a solution and didn't manage to find one. Could you please give me a hand with this?

Thank you in advance.