Currently, a Duration of minus [2 hours and 21 minutes] is formatted as "PT-2H-21M".
Another equivalent (and I would argue better) form would be "-PT2H21M".

Firstly, negative durations are not (I believe) part of the ISO 8601-1 standard.

The Java 8 Duration.parse method gives examples of valid input where both forms would be accepted but specifies

The leading plus/minus sign, and negative values for other units are not part of the ISO-8601 standard.¹

Negative durations seem to have been introduced in ISO 8601-2:2019, an extension to the original standard.

From my understanding, this version of the standard introduces the concept of a "negative duration", which is simply a "positive duration" prefixed with a single - sign (section 4.4.1.9²). However, at the same time, they also introduced a "composite representation" which indeed encompasses the current brick/date-time representation PT-2H-21M (sections 11.2 and 11.3.2²).

I also found a thread from the Ecma TC39 (JavaScript) "Temporal API" design group discussing the changes from ISO 8601-2:2019 and the impact on the proposal. This made me look at the Temporal.Duration documentation from MDN to find out their up-to-date conclusions.

There, we can see that their definition of the ISO 8061 format for Duration is ±P nY nM nW nD T nH nM nS³.
And surely, this is reflected in the Firefox JS console:

> Temporal.Duration.from("-PT2H21M")
< Temporal.Duration -PT2H21M

> Temporal.Duration.from("PT-2H-21M")
Uncaught RangeError: can't parse duration: missing duration digits

So, while the current representation PT-2H-21M seems valid as an ISO 8601-2:2019 "composite representation", it looks like the standard encourages instead the -PT2H21M format (simply "negative duration").

And anyway, while Java seems happy with either, Javascript Temporal (still experimental) only accepts -PT2H21M. I believe JS compatibility for the output of a PHP library to be an important factor to consider.

We would be happy to submit a patch to Duration::toISOString(). What do you think?