Parse xml with namespaces with SimpleXMLparser php

You should not extract the namespace URIs from the document. The namespace URI is a unique string defining the XML semantic the tag is part of. In other words the URI identifies the namespace - not the alias. Your XML is a good example for that, because it has Point elements in two different namespaces.

• p:Point resolves to {http://example.org}Point
• gml:Point resolves to {http://www.opengis.net/gml}Point

The namespace prefixes like p and gml are aliases to make a document smaller and more readable. They are only valid for the element and its children. They can be redefined on any element and they are optional for elements. More important they are only valid for the document. The following three examples all resolve to {http://example.org}Point.

• Prefix "p": <p:Point xmlns:p="http://example.org"/>
• Prefix "example": <example:Point xmlns:example="http://example.org"/>
• No prefix: <Point xmlns="http://example.org"/>

To read XML you define own prefixes for the namespaces and use them with Xpath or you use the namespace aware variants of the DOM methods like getAttributeNS(). Xpath is by a long way the more elegant solution. You can use the prefixes from the document or different ones.

SimpleXML requires you to register the namespaces on every SimpleXMLElement on that you're calling xpath(). To avoid code duplication I suggest to define an array with the namespaces and a function that registers them.

// define an array with the namespaces you're using
// the keys are your aliases - independent of the XML document.
$xmlns = [
  'gml' => 'http://www.opengis.net/gml',
  'p' => 'http://example.org'
];
// wrap the array in a function
$registerNamespaces = static function(SimpleXMLElement $target) use ($xmlns) {
  foreach ($xmlns as $alias => $uri) {
    $target->registerXPathNamespace($alias, $uri);
  }
};

$element = simplexml_load_string($content);
$registerNamespaces($element);

$result = [];
// use your prefixes to references the namespaces in Xpath
$positions = $element->xpath('//p:Point[1]//gml:pos');
foreach ($positions as $pos) {
  $result[] = (string)$pos;
}

var_dump($result);

Output: https://eval.in/159739

array(5) {
  [0]=>
  string(23) "-3.84307585 43.46031547"
  [1]=>
  string(23) "-3.84299411 43.46018513"
  [2]=>
  string(23) "-3.84299935 43.45998723"
  [3]=>
  string(23) "-3.84309913 43.46054546"
  [4]=>
  string(23) "-3.84307585 43.46031547"
}