Pernicious "trying to get property of a non-object" PHP error - php

Question

I am parsing an XML file in PHP as a DOMDocument. This is part of relatively stable code that I wrote awhile ago to parse our XML and has been working fine until recently. In the middle of a bunch of similar statements, I have the following line of code:
$appDomain = $domDoc->getElementsByTagName("appDomain")->item(0)->textContent;
This method throws an error "Trying to get property of non-object", but the code still goes through and works fine.
What's driving me nuts is the following. Using copy/paste I added the following line.
echo "". $domDoc->getElementsByTagName("appDomain")->item(0)->textContent;
$appDomain = $domDoc->getElementsByTagName("appDomain")->item(0)->textContent;
So now I'm running exactly the same code twice, one time echoing it and one time storing it. The echo correctly prints the value of the tag (which is Extract) from my XML, but I still get the error, and the error line is the assignment to $appDomain line, not the echo line.
Furthermore, if I add
echo $appDomain;
immediately after this call, it prints the correct value "Extract" as if no error has occurred. If I comment out the assignment to $appDomain, I appropriately get an "Undefined variable" error on the echo $appDomain line (I just did this to check that $appDomain isn't set somewhere else). If I then add a hardcoded $appDomain, like so:
echo "". $domDoc->getElementsByTagName("appDomain")->item(0)->textContent;
//$appDomain = $domDoc->getElementsByTagName("appDomain")->item(0)->textContent;
$appDomain = "Extract";
echo $appDomain;
Now the code prints "ExtractExtract" appropriately but I get the trying to get a property error on the FIRST echo line.
WHAT THE HECK IS GOING ON? The tag is in the XML (has been for awhile now, and this code has been working fine). The code correctly gets the element and gets it's text content and places it correctly in the variable $appDomain. But it still throws an error.
Also, the following var_dumps seem to behave exactly as they should:
var_dump($domDoc->getElementsByTagName("appDomain"));
echo "<br><br>";
var_dump($domDoc->getElementsByTagName("appDomain")->item(0));
echo "<br><br>";
var_dump($domDoc->getElementsByTagName("appDomain")->item(0)->textContent);
I get the following:
object(DOMNodeList)#3 (1) { ["length"]=> int(1) }
object(DOMElement)#2 (18) { ["tagName"]=> string(9) "appDomain" ["schemaTypeInfo"]=> NULL ["nodeName"]=> string(9) "appDomain" ["nodeValue"]=> string(7) "Extract" ["nodeType"]=> int(1) ["parentNode"]=> string(22) "(object value omitted)" ["childNodes"]=> string(22) "(object value omitted)" ["firstChild"]=> string(22) "(object value omitted)" ["lastChild"]=> string(22) "(object value omitted)" ["previousSibling"]=> string(22) "(object value omitted)" ["nextSibling"]=> string(22) "(object value omitted)" ["attributes"]=> string(22) "(object value omitted)" ["ownerDocument"]=> string(22) "(object value omitted)" ["namespaceURI"]=> NULL ["prefix"]=> string(0) "" ["localName"]=> string(9) "appDomain" ["baseURI"]=> string(62) "/Applications/MAMP/htdocs/Tmp/bowersjc/Experiment33/config.xml" ["textContent"]=> string(7) "Extract" }
string(7) "Extract" Extract
But I get the error on the last var_dump (and if I comment it out, I get no errors).
Again, WHAT THE HECK IS GOING ON?

Answer 1

I ran into the same error in a recursive class method, where the object was definitely an object, but the error (warning) kept happening.
First you can double check that it's really an object with is_object($el)
Sometimes recursion (xml parsers are recursive) and passing arguments into object methods can cause problems. It may have to do with the argument ->item(0). Try ->item()[0] for element 0, instead of argument 0.