Issue appending to xml from html using SimpleXML - php

Question

I am trying to do a simple append to an existing XML document using SimpleXML in PHP. I have an HTML form that calls a PHP script that tries to append data to an already existing XML document in the same directory.
I have tried the basic implementation of this, but I keep getting the follow error:
Warning: SimpleXMLElement::addChild(): Cannot add child. Parent is
not a permanent member of the XML tree in
/the/directory/to/the/site/generate.php on line
9
Fatal error: Uncaught Error: Call to a member function addChild() on
null...
Here is the already existing XML file that is being used:
<?xml version="1.0" encoding="UTF-8"?>
<tasks>
<users/>
<taskList>
<tasks id="1234">
<activities>
<activity/>
</activities>
</task>
</taskList>
</tasks>
Here is the PHP code:
<?php
$file = 'tasks.xml';
$xml = simplexml_load_file($file);
$activities = $xml->activities;
$activity = $activities->addChild('activity');
$activity->setAttribute('id', '45678');
$activity->addChild('assigned', 'Jon');
$activity->addChild('priority', 'low');
$xml->asXML($file);
I am hard coding the values in just to get the append to work, but eventually these values will be from a submitted html form.
Any ideas as to why this is failing?

Answer 1

There are a couple of problems with your code. Firstly your XML is invalid, your close tag should be </tasks>.
When you try and get the <activities> tag in
$activities = $xml->activities;
this is trying to find the tag just off the root of the document, you could use the full path
$activities = $xml->taskList->tasks->activities;
or use (I have in this code) XPath to find it, this also allows you to pick out (if neccessary) which <tasks> tag you use depending of id.
$activities = $xml->xpath("//activities")[0];
$activity = $activities->addChild('activity');
$activity->addAttribute('id', '45678');
$activity->addChild('assigned', 'Jon');
$activity->addChild('priority', 'low');
You also use setAttribute() which doesn't exist - as the code shows it is addAttribute().