But simplexml_load_string() loses the period-name value

It's not simplexml_load_string() that loses the attribute, but the conversion to JSON. The "problem" is that you can't necessarily treat the SimpleXMLElement "Object" as an ordinary object - under the hood it's a resource. You can't simply dump the object as a string, consequently it is non-serializable. All the information is there, you just need to interact with it as a SimpleXMLElement object.

For example, to access the "start-valid-time" elements with the "period-name" attributes in your example you can do something like:

$contents = <<<EOF
<data foo="bar">
 <time-layout foo="bar">
  <start-valid-time period-name="Tonight">2020-01-22T18:00:00-05:00</start-valid-time>
  <start-valid-time period-name="Tomorrow">2020-01-23T06:00:00-05:00</start-valid-time>
 </time-layout>
</data>
EOF;

$XML = simplexml_load_string($contents);
$result = array();
foreach ($XML->{'time-layout'}->{'start-valid-time'} as $startValidTime) {
  $result[] = array (
    // Cast to "string" to extract values
    'period-name' => (string) $startValidTime['period-name'],
    'value' => (string) $startValidTime,
  );
}
print_r($result);

Will output:

Array
(
  [0] => Array
    (
      [period-name] => Tonight
      [value] => 2020-01-22T18:00:00-05:00
    )

  [1] => Array
    (
      [period-name] => Tomorrow
      [value] => 2020-01-23T06:00:00-05:00
    )
)

I appreciate the help, @w3dk! But unfortunately, when I simply do this:

$xml = simplexml_load_string($contents);
print_r($xml);

there's nothing called period-name in the results. For whatever reason, it's disappearing after simplexml_load_string(), regardless of JSON.

Could it be my version of PHP?

Here's the entire array for [time-layout] so you can see:

           [time-layout] => Array
            (
              [0] => SimpleXMLElement Object
                (
                  [@attributes] => Array
                    (
                      [time-coordinate] => local
                      [summarization] => 12hourly
                    )

                  [layout-key] => k-p12h-n14-1
                  [start-valid-time] => Array
                    (
                      [0] => 2020-01-23T20:00:00-05:00
                      [1] => 2020-01-24T06:00:00-05:00
                      [2] => 2020-01-24T18:00:00-05:00
                      [3] => 2020-01-25T06:00:00-05:00
                      [4] => 2020-01-25T18:00:00-05:00
                      [5] => 2020-01-26T06:00:00-05:00
                      [6] => 2020-01-26T18:00:00-05:00
                      [7] => 2020-01-27T06:00:00-05:00
                      [8] => 2020-01-27T18:00:00-05:00
                      [9] => 2020-01-28T06:00:00-05:00
                      [10] => 2020-01-28T18:00:00-05:00
                      [11] => 2020-01-29T06:00:00-05:00
                      [12] => 2020-01-29T18:00:00-05:00
                      [13] => 2020-01-30T06:00:00-05:00
                    )
                )

when I simply do this:

$xml = simplexml_load_string($contents);
print_r($xml);

there's nothing called period-name in the results.

Yes, that is expected. Because you can't simply print_r() a SimpleXMLElement object and expect to see the entire contents of that object. (That might work for "simple" XML files, but not here.) When you call print_r() on an object, PHP must convert that object into a human-readable string representation. This is not always possible with "complex" objects. SimpleXMLElement is one such object.

For example, once you've created a SimpleXMLElement object with simplexml_load_string(), try calling the asXML() method and look at the result - you should see the complete XML document (regenerated from the data as read in).

The code I posted above is a working example using the XML snippet you posted - it reads the "period-name" attribute and value from the nested "start-valid-time" elements and returns an array. Exactly how you return that information is up to you. (I think in your other thread you were wanting to index by the value - timestamp?)

Well slap my butt and call me Sally... you're right :-) I had copied your exact code earlier and just came back with an empty array, but I must have made a typo or something because it worked like you said this time!

That's awesome, I really appreciate it!

So I've been playing around, and using your code as a jumping point I was able to really simplify a lot of what I had done. So now I've gotten to:

foreach ($XML->{'data'}->{0}->{'time-layout'}->{0}->{'start-valid-time'} as $key) {

 // 2020-01-25T15:00:00-05:00 => This Afternoon
 echo "$key => {$key['period-name']}\n";

 $thisDate = strtotime($key);

 // $forecast['period-name']['0123456789'] => 'This Afternoon'
 $forecast['period-name'][$thisDate] = $key['period-name'];
}

print_r($forecast);

The echo statement prints exactly what I'm expecting:

2020-01-25T15:00:00-05:00 => This Afternoon

but print_r($forecast) doesn't; instead, I'm getting:

[period-name] => Array
    (
      [1579982400] => SimpleXMLElement Object
        (
          [0] => This Afternoon
        )

I don't understand, why does $forecast['period-name'] print as a string, but the value is set as an object?

I found that if I set it like this, though, then it sets as a string:

$forecast['period-name'][$thisDate] = "{$key['period-name']}";

Why?

Because internally $key['period-name'] is a SimpleXMLElement object. To access the value of that element/attribute, you need to cast it to a "string". You could call it a quirk of SimpleXML. Although internally, there's probably a __toString() magic method that does the "magic".

When you echo the value or surround the variable in double quotes (variable parsing) then PHP implicitly type converts the variable to a string.

However, it is preferable to use explicit type casting when possible, as in my code above. For example:

$thisDate = strtotime((string) $key);
$forecast['period-name'][$thisDate] = (string) $key['period-name'];

This is a more deliberate action, easier to read, less chance of bugs etc.