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

Which version of php do you use? With my php5.3.5/win32 it prints `true`. — VolkerK, Nov 08 '11 at 12:19
The second one throws the following: `Fatal error: Uncaught exception 'Exception' with message 'String could not be parsed as XML' in /t.php:2` here: http://codepad.org/KIbscPGh — Brendan Bullen, Nov 08 '11 at 12:20
possible duplicate of [What do PHP closures return in IF statements?](http://stackoverflow.com/questions/8040067/what-do-php-closures-return-in-if-statements) — hakre, Nov 08 '11 at 12:23
Am I right in thinking the edits to this question have changed the meaning? The OP originally posted the second block with an empty string and now it has become ``? — Brendan Bullen, Nov 08 '11 at 12:24
@BrendanBullen - There was never an empty string. It remained invisible because code wasn't formatted as code. — Álvaro González, Nov 08 '11 at 12:30
@ÁlvaroG.Vicario Ah that makes sense! Viewing the source of the edits reveals that. Thanks — Brendan Bullen, Nov 08 '11 at 14:20

score 1 · Accepted Answer · answered Nov 08 '11 at 12:57

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);

hakre · Answer 2 · 2011-11-08T12:34:56.530

Objects in PHP (5+) evaluate to TRUE, always.

The one exception are SimpleXML objects created from empty tags - see http://php.net/manual/en/language.types.boolean.php

$xml = new SimpleXMLElement('<b></b>');

qualifies as such a FALSE SimpleXML object (FALSE === (bool) $xml;).

However

$xml = new SimpleXMLElement('');

Throws an exception and will halt your program if not caught. If caught, $xml is not set.

score 0 · Answer 3 · answered Nov 08 '11 at 12:33

You can find everything that evaluates to true/false on http://www.php.net/manual/en/language.types.boolean.php (find "When converting to boolean, the following values are considered FALSE:")

Creating a SimpleXMLElement object will throw an exception if it fails. Following code should do what you need:

try
{
    $xml=new SimpleXMLElement('');
    var_dump($xml);
    echo 'true';
}
catch
{
    echo 'false';
}

To evaluate to true/false, you might be looking into ===, to explicitly evaluate to false.

$test = '';
echo $test ? 'true' : 'false'; // will output false
echo $test !== false ? 'true' : 'false'; // will output true

Why does a SimpleXML object evaluates to boolean TRUE?

3 Answers3