一步分配xpath-> query和read length

Question

Why is this not allowed? (I get a syntax error at the ->length):

($x=$xpath->query($s))->length

According to the http://php.net/manual/en/language.operators.assignment.php the value of an assignment expression is the value assigned, but it does not seem to work when assigning the result of a xpath->query.

I want to use the expression in a while loop, where I'm assigning the query result and checking for zero-length in one step:

// $xpath is a DOMXPATH object
while(($x=$xpath->query($s))->length){
    // do something with $x
}

Answer 1

I'm not in anyway a PHP expert, but from what I observed, it is combination of the parentheses and object operator (->) that triggers the error. The assignment expression itself correctly returns an instance of DOMNodeList, as we expect given the linked documentation :

$xml = "<person><name>John-Doe</name></person>";
$doc = new DOMDocument();
$doc->loadXML($xml);
$xpath = new DOMXpath($doc);

$test = ($x = $xpath->query("/person/foo"));
//$test contains instance of `DOMNodeList` on which we can use `->` to get the length
echo $test->length;

//but using `->` on parentheses triggers the initial error :
echo ($test)->length;

To achieve the actual goal, you maybe able to use plain for loop, which provides more flexibility in controlling the loop, instead of using while. Something along the lines of the following snippet :

for($x=$xpath->query($s);
    $x->length;
    $x=$xpath->query($s))
{
    //do something with $x
}

I assume that $xpath->query($s) result changes in the next iterations, in a way that $x->length value decreases, so that the loop will eventually terminates.