使用PHP解析XML文件（cs-s75：David Malan's）

I am making a PHP project for a Pizza Shop [This is project-0 in David Malan's course CS-S75 Building Dynamic Websites]. And the Code that I have to write must be eXtensible. That is, if the pizza shop's owner wants to add a new category, he should be able to do that pretty easily and my PHP code must accommodate those changes in the XML file without writing any new code.

For my code to be extensible though, I need some methods for filtering the XML data.
For instance inside the root node <menu>, I have child nodes item that have attributes like

    <item name="Pizzas">
        <category name="Onions">
        </category>
    </item>

    <item name="Salads">
        <category name = "Garden">
        </category>
    </item>

and there are ten item tags in total.

What I want to do is this: if the user wants to purchase the salads, I would want to filter the XML DOM tree the following way:

// $_POST['selected'] has a value of 'Salads' stored in it
$selected = $_POST['selected'] 
$dom = simple_xml_loadfile("menu.xml")
foreach ($dom -> xpath("menu/item[@name = $selected ]" as $item))
{
   echo $item -> category['name'].'<br />';
}

And it should print Garden and any other item that is subsequently added to the Salads category.The problem occurs with the menu/item[@name = $selected ] because this is probably not a proper method for comparing the attribute (Note that attribute comparison like this in XML requires single equal sign and not double equal).And obviously menu/item[@name = $_POST['selected']] doesn't work either. What works is @name = "Salads" and of course this kills the whole purpose of the extensiblity of XML and dynamism of PHP.
Please help!

Let's get all category nodes that belong to a parent node that has a name attribute of your choosing:

Also note that the function name is simplexml_load_file and not simple_xml_loadfile

foreach ($dom->xpath('item[@name="' . $selected . '"]/category') as $item)
{
   echo $item->attributes()->{'name'}. PHP_EOL;
}

Also note the usage of single vs. double quotes to enclose the attribute value.

For reference, this is the xml structure I used for testing:

<menu>
<item name="Pizzas">
    <category name="Onions"></category>
</item>
<item name="Salads">
    <category name = "Garden"></category>
    <category name = "Cesar"></category>
    <category name = "Onion and Tomato"></category>
</item>
</menu>