'Serialization of 'SimpleXMLElement' is not allowed when saving in Wordpress post_meta

I am working on an amazon affiliate wordpress page.
For that I am using the aws_signed_request function to get the price and link from amazon.

Here is the aws_signed_request function returning the xml:

function aws_signed_request($region, $params, $public_key, $private_key, $associate_tag) {
$method = "GET";
$host = "ecs.amazonaws.".$region;
$uri = "/onca/xml";

$params["Service"] = "AWSECommerceService";
$params["AWSAccessKeyId"] = $public_key;
$params["AssociateTag"] = $associate_tag;
$params["Timestamp"] = gmdate("Y-m-d\TH:i:s\Z");
$params["Version"] = "2009-03-31";

ksort($params);

$canonicalized_query = array();

foreach ($params as $param=>$value)
{
$param = str_replace("%7E", "~", rawurlencode($param));
$value = str_replace("%7E", "~", rawurlencode($value));
$canonicalized_query[] = $param."=".$value;
}

$canonicalized_query = implode("&", $canonicalized_query);

$string_to_sign = $method."\n".$host."\n".$uri."\n".
$canonicalized_query;

/* calculate the signature using HMAC, SHA256 and base64-encoding */
$signature = base64_encode(hash_hmac("sha256",
$string_to_sign, $private_key, True));

/* encode the signature for the request */
$signature = str_replace("%7E", "~", rawurlencode($signature));

/* create request */
$request = "http://".$host.$uri."?".$canonicalized_query."&Signature=".$signature;

/* I prefer using CURL */
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$request);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 15);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);

$xml_response = curl_exec($ch);

if ($xml_response === False)
{
return False;
}
else
{
$parsed_xml = @simplexml_load_string($xml_response);
return ($parsed_xml === False) ? False : $parsed_xml;
}
}


After that I get the asin from the post and generate the link and price

global $post;
$asin = get_post_meta($post->ID, 'ASIN', true);

$public_key = 'xxxxxxxxxxx';
$private_key = 'xxxxxxxxxxx';
$associate_tag = 'xxxxxxxxxxx';

$xml = aws_signed_Request('de',
array(
"MerchantId"=>"Amazon",
"Operation"=>"ItemLookup",
"ItemId"=>$asin,
"ResponseGroup"=>"Medium, Offers"),
$public_key,$private_key,$associate_tag);

$item = $xml->Items->Item;

$link = $item->DetailPageURL;
$price_amount = $item->OfferSummary->LowestNewPrice->Amount;
if ($price_amount > 0) {
$price_rund = $price_amount/100;
$price = number_format($price_rund, 2, ',', '.');
} else {
$price= "n.v.";
}

This all works pretty good when I echo the $link and $price. But I want to save the values in the custom fields of the wordpress post so I don't have to run the function every time.

update_post_meta($post->ID, 'Price', $price);
update_post_meta($post->ID, 'Link', $link);

This adds the price as the correct value, but when I want to add the link I get this error message:

> Uncaught exception 'Exception' with message 'Serialization of
> 'SimpleXMLElement' is not allowed' in...

But when I remove the $parsed_xml=... function, it saves an empty value.

(Nearly) everything returned when you are traversing a SimpleXML object is actually another SimpleXML object. This is what lets you write `$item->OfferSummary->LowestNewPrice->Amount`: requesting `->OfferSummary` on the `$item` object returns an object representing the `OfferSummary` XML node, so you can request `->LowestNewPrice` on that object, and so on. Note that this applies to attributes too - `$someNode['someAttribute']` will be an object, not a string!

In order to get the string content of an element or attribute, you have to "cast" it, using the syntax `(string)$variable`. Sometimes, PHP will know you meant to do this, and do it for you - for instance when using `echo` - but in general, it's good practice to **always cast to string manually** so that you won't have any surprises if you change your code later. You can also cast to an integer using `(int)`, or a float using `(float)`.

The second part of your problem is that SimpleXML objects are stored rather specially in memory, and can't be "serialized" (i.e. turned into a string that completely describes the object). This means that if you try to save them into a database or session, you will get the error you're seeing. If you actually wanted to save a whole block of XML, you could use `$foo->asXML()`.

So, in short:

- use `$link = (string)$item->DetailPageURL;` to get a string rather than an object
- use `update_post_meta($post->ID, 'ItemXML', $item->asXML());` if you ever want to store the whole item