Unable to Parse XML having multiple Namespace Using PHP

5F is an invalid tag name - they are not allowed to start with digits. The parser will throw warnings if you load this (invalid) XML.

Warning: simplexml_load_string(): namespace error : Failed to parse QName 'FORMM:' in /in/HHTnS on line 33

Warning: simplexml_load_string():     <FORMM:5F> in /in/HHTnS on line 33

Warning: simplexml_load_string():            ^ in /in/HHTnS on line 33

So you will need some error handling for this and/or have the XML fixed.

However LibXML (the library behind SimpleXML and DOM) tolerates it. So you can still read it.

Don't fetch namespaces from the document. This is method for debugging or generic transformations. The namespace URI is the identifying value of a namespace. Namespaces prefixes can change at any element in an XML. Define/register you own prefixes for the Xpath expressions and use the Namespace URI in the method calls.

print_r(), var_dump(), ... are using SimpleXMLs mapping of XML elements. This is a compromise and has limits (especially with namespaces). Try SimpleXMLElement::asXML() to debug a SimpleXMLElement instance.

You can still access the elements using property syntax, but you might need to use an SimpleXMLElement::children() to specify the namespace.

Here is an example based on your question:

$xml = <<<'XML'
<?xml version="1.0" encoding="utf-8"?>
<RETURN:RT 
    xmlns:FORMM="http://example.com/5"
    xmlns:Form="http://example.com/master"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:RETURN="http://example.com">
    <FORMM:5F>
        <Form:Info>
            <Form:SWCreatedBy>SW10002087</Form:SWCreatedBy>
        </Form:Info>
   </FORMM:5F>
</RETURN:RT>
XML;

// define namespaces you're going to use
$xmlns = [
  'five' => 'http://example.com/5',
  'master' => 'http://example.com/master',
  'return' => 'http://example.com',
];

// a function to apply all of them to a SimpleXMLElement
function registerXpathNamespaces(SimpleXMLElement $target, array $xmlns) {
    foreach ($xmlns as $prefix => $namespaceURI) {
        $target->registerXpathNamespace($prefix, $namespaceURI);
    }
}

$rt = simplexml_load_string($xml);
registerXpathNamespaces($rt, $xmlns);
$list = $rt->xpath('//return:RT/*');
foreach ($list as $fiveF) {
    // serialize the node into a string for debugging
   var_dump($fiveF->asXML());
   // use the namespace list to access child elments in a specific namespace
   var_dump(
       (string)$fiveF->children($xmlns['master'])->Info->SWCreatedBy
   );
}

Output:

string(130) "<FORMM:5F>
        <Form:Info>
            <Form:SWCreatedBy>SW10002087</Form:SWCreatedBy>
        </Form:Info>
   </FORMM:5F>"
string(10) "SW10002087"

You need to register the namespaces on each SimpleXMLElement instance you are calling xpath() on.

In DOM, here is a separate Xpath object, so you only need to register the namespaces once. Additionally DOMXpath::evaluate() can return scalar values (not just node lists). This it how the example would look with DOM:

// define namespaces you're going to use
$xmlns = [
  'five' => 'http://example.com/5',
  'master' => 'http://example.com/master',
  'return' => 'http://example.com',
];

$document = new DOMDocument();
$document->loadXML($xml);
$xpath = new DOMXpath($document);
foreach ($xmlns as $prefix => $namespaceURI) {
    $xpath->registerNamespace($prefix, $namespaceURI);
}

$list = $xpath->evaluate('//return:RT/*');
foreach ($list as $fiveF) {
    // serialize the node into a string for debugging
   var_dump($document->saveXML($fiveF));
   // directly fetch the value using a string cast in Xpath
   var_dump(
       $xpath->evaluate(
           'string(master:Info/master:SWCreatedBy)', 
           $fiveF
       )
   );
}

Using DOM you can take a look how the parser reads the broken tag.

// the RETURN:RT is valid - the parser can match it to the namespace
var_dump($document->documentElement->localName, $document->documentElement->namespaceURI);
foreach ($list as $fiveF) {
   // the "5F" or "FORMM:5F" is invalid - the parser will not match it
   var_dump($fiveF->localName, $fiveF->namespaceURI);
}

string(2) "RT"
string(18) "http://example.com"
string(8) "FORMM:5F"
NULL

So it keeps the alias/prefix as a part of the name and ignores the namespace.