Three line xml2array:

<?php

$xml = simplexml_load_string($xmlstring);
$json = json_encode($xml);
$array = json_decode($json,TRUE);

?>

Ta da!

To add to what others have said, you can't directly put a $_GET or $_POST value into a variable then into an attribute using SimpleXML. You must first convert it to an integer.

This will NOT work

<?php
      $page_id = $_GET['id'];
      echo $xml->page[$page_id]
?>

You will get something like:
Notice: Trying to get property of non-object in /Applications/MAMP/htdocs/mysite/index.php on line 10

However, this WILL work and is much simpler then using (string) or other methods.
<?php
   $page_id = intval($_GET['id']);
   echo $xml->page[$page_id]
?>

Storing SimpleXMLElement values in $_SESSION does not work. Saving the results as an object or individual elements of the object will result in the dreaded "Warning: session_start() [function.session-start]: Node no longer exists" error.

For example, this does not work:

    $xml  = new SimpleXMLElement($page);
    $country  = $xml->Response->Placemark->AddressDetails->Country->CountryNameCode;
    $_SESSION['country'] = $country;

This will work:

    $_SESSION['country'] = (string) $country;

Here is a recursive function that will convert a given SimpleXMLElement object into an array, preserving namespaces and attributes.

<?php
function xmlObjToArr($obj) {
        $namespace = $obj->getDocNamespaces(true);
        $namespace[NULL] = NULL;
        
        $children = array();
        $attributes = array();
        $name = strtolower((string)$obj->getName());
        
        $text = trim((string)$obj);
        if( strlen($text) <= 0 ) {
            $text = NULL;
        }
        
        // get info for all namespaces
        if(is_object($obj)) {
            foreach( $namespace as $ns=>$nsUrl ) {
                // atributes
                $objAttributes = $obj->attributes($ns, true);
                foreach( $objAttributes as $attributeName => $attributeValue ) {
                    $attribName = strtolower(trim((string)$attributeName));
                    $attribVal = trim((string)$attributeValue);
                    if (!empty($ns)) {
                        $attribName = $ns . ':' . $attribName;
                    }
                    $attributes[$attribName] = $attribVal;
                }
                
                // children
                $objChildren = $obj->children($ns, true);
                foreach( $objChildren as $childName=>$child ) {
                    $childName = strtolower((string)$childName);
                    if( !empty($ns) ) {
                        $childName = $ns.':'.$childName;
                    }
                    $children[$childName][] = xmlObjToArr($child);
                }
            }
        }
        
        return array(
            'name'=>$name,
            'text'=>$text,
            'attributes'=>$attributes,
            'children'=>$children
        );
    }
?>

Here are two quick and dirty functions that use SimpleXML to detect if a feed xml is RSS or ATOM:

<?php
function is_rss($feedxml) {
    @$feed = new SimpleXMLElement($feedxml);

    if ($feed->channel->item) {
        return true;
    } else {
        return false;
    }
}

function is_atom($feedxml) {
    @$feed = new SimpleXMLElement($feedxml);

    if ($feed->entry) {
        return true;
    } else {
        return false;
    }
}
?>

The functions take in the full text feed (retrieved via cURL, for example) and return a true or a false based on the result.

Simple means simple.  If you know the structure and just want the value of a tag:

<?php
$xml = simplexml_load_file($xmlfile);
print $xml->City->Street->Address->HouseColor;
?>

Warning, numbers can come out as strings, empty elements like <HouseColor></HouseColor> come out as array(0)

while using simple xml and get double or float int value from xml object for using math operations (+ * - / ) some errors happens on the operation, this is because of simple xml returns everythings to objects. 
exmple;

<?php

$name = "somestring";
$size = 11.45;
$xml = '
<xml>
 <name>somestring</name>
 <size>11.45</size>
</xml>';

 
$xmlget = simplexml_load_string($xml)

echo $xml->size*2;    // 20  its false
// ($xml->size is an object (int)11 and  (45) )

// this is true
echo $size*2;            // 22.90 
echo (float)$size*2;   // 22.90 
?>

Wrapper XMLReader class, for simple SAX-reading huge xml:
https://github.com/dkrnl/SimpleXMLReader

Usage example: http://github.com/dkrnl/SimpleXMLReader/blob/master/examples/example1.php

<?php

/**
 * Simple XML Reader
 *
 * @license Public Domain
 * @author Dmitry Pyatkov(aka dkrnl) <[email protected]>
 * @url http://github.com/dkrnl/SimpleXMLReader
 */
class SimpleXMLReader extends XMLReader
{

    /**
     * Callbacks
     *
     * @var array
     */
    protected $callback = array();

    /**
     * Add node callback
     *
     * @param  string   $name
     * @param  callback $callback
     * @param  integer  $nodeType
     * @return SimpleXMLReader
     */
    public function registerCallback($name, $callback, $nodeType = XMLREADER::ELEMENT)
    {
        if (isset($this->callback[$nodeType][$name])) {
            throw new Exception("Already exists callback $name($nodeType).");
        }
        if (!is_callable($callback)) {
            throw new Exception("Already exists parser callback $name($nodeType).");
        }
        $this->callback[$nodeType][$name] = $callback;
        return $this;
    }

    /**
     * Remove node callback
     *
     * @param  string  $name
     * @param  integer $nodeType
     * @return SimpleXMLReader
     */
    public function unRegisterCallback($name, $nodeType = XMLREADER::ELEMENT)
    {
        if (!isset($this->callback[$nodeType][$name])) {
            throw new Exception("Unknow parser callback $name($nodeType).");
        }
        unset($this->callback[$nodeType][$name]);
        return $this;
    }

    /**
     * Run parser
     *
     * @return void
     */
    public function parse()
    {
        if (empty($this->callback)) {
            throw new Exception("Empty parser callback.");
        }
        $continue = true;
        while ($continue && $this->read()) {
            if (isset($this->callback[$this->nodeType][$this->name])) {
                $continue = call_user_func($this->callback[$this->nodeType][$this->name], $this);
            }
        }
    }

    /**
     * Run XPath query on current node
     *
     * @param  string $path
     * @param  string $version
     * @param  string $encoding
     * @return array(SimpleXMLElement)
     */
    public function expandXpath($path, $version = "1.0", $encoding = "UTF-8")
    {
        return $this->expandSimpleXml($version, $encoding)->xpath($path);
    }

    /**
     * Expand current node to string
     *
     * @param  string $version
     * @param  string $encoding
     * @return SimpleXMLElement
     */
    public function expandString($version = "1.0", $encoding = "UTF-8")
    {
        return $this->expandSimpleXml($version, $encoding)->asXML();
    }

    /**
     * Expand current node to SimpleXMLElement
     *
     * @param  string $version
     * @param  string $encoding
     * @param  string $className
     * @return SimpleXMLElement
     */
    public function expandSimpleXml($version = "1.0", $encoding = "UTF-8", $className = null)
    {
        $element = $this->expand();
        $document = new DomDocument($version, $encoding);
        $node = $document->importNode($element, true);
        $document->appendChild($node);
        return simplexml_import_dom($node, $className);
    }

    /**
     * Expand current node to DomDocument
     *
     * @param  string $version
     * @param  string $encoding
     * @return DomDocument
     */
    public function expandDomDocument($version = "1.0", $encoding = "UTF-8")
    {
        $element = $this->expand();
        $document = new DomDocument($version, $encoding);
        $node = $document->importNode($element, true);
        $document->appendChild($node);
        return $document;
    }

}
?>

The BIGGEST differece between an XML and a PHP array is that in an XML file, the name of elements can be the same even if they are siblings, eg. "<pa><ch /><ch /><ch /></pa>", while in an PHP array, the key of which must be different.

I think the array structure developed by svdmeer can fit for XML, and fits well.

here is an example array converted from an xml file:
array(
"@tag"=>"name",
"@attr"=>array(
    "id"=>"1","class"=>"2")
"@text"=>"some text",
)

or if it has childrens, that can be:

array(
"@tag"=>"name",
"@attr"=>array(
    "id"=>"1","class"=>"2")
"@items"=>array(
    0=>array(
        "@tag"=>"name","@text"=>"some text"
    )
)

Also, I wrote a function that can change that array back to XML.

<?php
function array2XML($arr,$root) {
$xml = new SimpleXMLElement("<?xml version=\"1.0\" encoding=\"utf-8\" ?><{$root}></{$root}>"); 
$f = create_function('$f,$c,$a',' 
        foreach($a as $v) {
            if(isset($v["@text"])) {
                $ch = $c->addChild($v["@tag"],$v["@text"]);
            } else {
                $ch = $c->addChild($v["@tag"]);
                if(isset($v["@items"])) {
                    $f($f,$ch,$v["@items"]);
                }
            }
            if(isset($v["@attr"])) {
                foreach($v["@attr"] as $attr => $val) {
                    $ch->addAttribute($attr,$val);
                }
            }
        }');
$f($f,$xml,$arr);
return $xml->asXML();
}
?>

Here's a quick way to dump the nodeValues from SimpleXML into an array using the path to each nodeValue as key. The paths are compatible with e.g. DOMXPath. I use this when I need to update values externally (i.e. in code that doesn't know about the underlying xml). Then I use DOMXPath to find the node containing the original value and update it.

<?php
function XMLToArrayFlat($xml, &$return, $path='', $root=false)
{
    $children = array();
    if ($xml instanceof SimpleXMLElement) {
        $children = $xml->children();
        if ($root){ // we're at root
            $path .= '/'.$xml->getName();
        }
    }
    if ( count($children) == 0 ){
        $return[$path] = (string)$xml;
        return;
    }
    $seen=array();
    foreach ($children as $child => $value) {
        $childname = ($child instanceof SimpleXMLElement)?$child->getName():$child;
        if ( !isset($seen[$childname])){
            $seen[$childname]=0;
        }
        $seen[$childname]++;
        XMLToArrayFlat($value, $return, $path.'/'.$child.'['.$seen[$childname].']');
    }
} 
?>

Use like this:

<?php
$xml = simplexml_load_string(...some xml string...);
$xmlarray = array(); // this will hold the flattened data
XMLToArrayFlat($xml, $xmlarray, '', true);
?>

You can also pull multiple files in one array:

<?php
foreach($files as $file){
    $xml = simplexml_load_file($file);
    XMLToArrayFlat($xml, $xmlarray, $file.':', true);
}
?>
The respective filename/path is thus prefixed to each key.

If you tried to load an XML file with this, but the CDATA parts were not loaded for some reason, is because you should do it this way:

$xml = simplexml_load_file($this->filename, 'SimpleXMLElement', LIBXML_NOCDATA);

This converts CDATA to String in the returning object.

Here's a function I came up with to convert an associative array to XML.  Works for multidimensional arrays as well.

<?php
function assocArrayToXML($root_element_name,$ar)
{
    $xml = new SimpleXMLElement("<?xml version=\"1.0\"?><{$root_element_name}></{$root_element_name}>");
    $f = create_function('$f,$c,$a','
            foreach($a as $k=>$v) {
                if(is_array($v)) {
                    $ch=$c->addChild($k);
                    $f($f,$ch,$v);
                } else {
                    $c->addChild($k,$v);
                }
            }');
    $f($f,$xml,$ar);
    return $xml->asXML();
}
?>

dynamic sql in php using xml:

test.xml:
<?xml version="1.0" encoding="UTF-8"?>
<sql>
    <statement>
        SELECT * FROM USERS
        <call criteria="byId">WHERE id = %d</call>
        <call criteria="byUsername">WHERE username = "%s"</call>;
    </statement>
</sql>

index.php:
<?php
function callMe($param) {
    $search = array('byUsername' => 'dynsql');
    
    if (isset($search[$param[1]])) {
        return sprintf($param[2], $search[$param[1]]);
    }
    
    return "";
}

$xml = simplexml_load_file("test.xml");
$string = $xml->statement->asXML();
$string = preg_replace_callback('/<call criteria="(\w+)">(.*?)<\/call>/', 'callMe', $string);
$node = simplexml_load_string($string);
echo $node;
?>

obviously, this example can be improved [in your own code.]

Two lines xml2array:

<?php

$xml = simplexml_load_string($xmlstring);
$array = (array) $xml;

?>

I had a problem with simplexml reading nodes from an xml file. It always return an SimpleXML-Object but not the text inside the node.

Example:
<?xml version="1.0" encoding="UTF-8"?>
<Test>
    <Id>123</Id>
</Test>

Reading this xml into a variable called $xml and then doing the following
<?php
$myId = $xml->Id;
?>
Did not return 123 in $myId, but instead I got a SimpleXMLElement Object.

The solution is simple, when you know it. Use explicit string conversion.
<?php
$myId = (string)$xml->Id;
?>