How to check that an object is empty in PHP?

You can cast to an array and then check if it is empty or not

$arr = (array)$obj;
if (!$arr) {
    // do stuff
}

Edit: I didn't realize they wanted to specifically check if a SimpleXMLElement object is empty. I left the old answer below

Updated Answer (SimpleXMLElement):

For SimpleXMLElement:

If by empty you mean has no properties:

$obj = simplexml_load_file($url);
if ( !$obj->count() )
{
    // no properties
}

OR

$obj = simplexml_load_file($url);
if ( !(array)$obj )
{
    // empty array
}

If SimpleXMLElement is one level deep, and by empty you actually mean that it only has properties PHP considers falsey (or no properties):

$obj = simplexml_load_file($url);
if ( !array_filter((array)$obj) )
{
    // all properties falsey or no properties at all
}

If SimpleXMLElement is more than one level deep, you can start by converting it to a pure array:

$obj = simplexml_load_file($url);
// `json_decode(json_encode($obj), TRUE)` can be slow because
// you're converting to and from a JSON string.
// I don't know another simple way to do a deep conversion from object to array
$array = json_decode(json_encode($obj), TRUE);
if ( !array_filter($array) )
{
    // empty or all properties falsey
}

Old Answer (simple object):

If you want to check if a simple object (type stdClass) is completely empty (no keys/values), you can do the following:

// $obj is type stdClass and we want to check if it's empty
if ( $obj == new stdClass() )
{
    echo "Object is empty"; // JSON: {}
}
else
{
    echo "Object has properties";
}

Source: http://php.net/manual/en/language.oop5.object-comparison.php

Edit: added example

$one = new stdClass();
$two = (object)array();

var_dump($one == new stdClass()); // TRUE
var_dump($two == new stdClass()); // TRUE
var_dump($one == $two); // TRUE

$two->test = TRUE;
var_dump($two == new stdClass()); // FALSE
var_dump($one == $two); // FALSE

$two->test = FALSE;
var_dump($one == $two); // FALSE

$two->test = NULL;
var_dump($one == $two); // FALSE

$two->test = TRUE;
$one->test = TRUE;
var_dump($one == $two); // TRUE

unset($one->test, $two->test);
var_dump($one == $two); // TRUE