Please note that a valid SimpleXMLElement is typecasted to false.
<?php
$string = '<root><foo>abc</foo></root>';
$xml = DOMDocument::loadXML($string);
$xml = simplexml_import_dom($xml);
echo 'simplexml_import_dom';
var_dump($xml);
var_dump((bool)$xml);
$xml = simplexml_load_string('<root><foo>abc</foo></root>');
echo 'simplexml_load_string';
var_dump($xml);
var_dump((bool)$xml);
$xml = simplexml_load_file('test.xml');
echo 'simplexml_load_file';
var_dump($xml);
var_dump((bool)$xml);
?>
Outputs:
--START--
simplexml_import_dom
object(SimpleXMLElement)[2]
public 'foo' => 'abc'
false
simplexml_load_string
object(SimpleXMLElement)[1]
public 'foo' => 'abc'
false
simplexml_load_file
object(SimpleXMLElement)[2]
public 'foo' => 'abc'
false
--END--