Why does a SimpleXML object evaluates to boolean TRUE?

Question

I am a newcomer to PHP and SimpleXMl.

I would not expect false under either of these tests.

  $xml=new SimpleXMLElement('<a><b>123</b></a>');
  var_dump($xml);
  echo $xml ? "true": "false";

or

  $xml=new SimpleXMLElement('<a><b></b></a>');
  var_dump($xml);
  echo $xml ? "true": "false";

however the second returns false even though an XMLSimpleElement object is returned. I have the same issue with an xml doc with namespaces everywhere.

it means I cannot test for a failed XML parsing as if (!xml) returns false

but $xml->childen($namespace) does not.

Please advise TIA Ephraim

Answer 1

I can't reproduce your issue but I'll try to provide a couple of hints.

First, here's the rule when converting objects to booleans:

When converting to boolean, the following values are considered FALSE:
[...] the special type NULL (including unset variables)
[...] SimpleXML objects created from empty tags

Every other value is considered TRUE (including any resource).

Second, the SimpleXMLElement constructor always returns an object but it can throw a warning and an exception.

So these are the possibilities:

// Casts as TRUE because it's an object
$xml=new SimpleXMLElement('<a><b>123</b></a>');
var_dump($xml, (bool)$xml);
unset($xml);

// Casts as FALSE because it's an SimpleXMLElement object for an empty tag
$xml=new SimpleXMLElement('<a />');
var_dump($xml, (bool)$xml);
unset($xml);

// Casts as FALSE because the $xml variable was never set sucessfully so it's not even set
try{
    $xml=new SimpleXMLElement('');
}catch(Exception $e){
}
var_dump($xml, (bool)$xml); // Notice: Undefined variable: xml
unset($xml);