The minimum version currently is 5.5, with 7.4 confirmed working.

You can try 8.x version PHP, and I'm successfully running Arcanist using PHP 8.1, but Phorge might be a different story.

PHP 8.0 works perfectly fine 8.1 isn’t fully supported.

Probably worthwhile to note, for those tracking this Q as well as anyone finding this via web search - as of recently, the brackets have moved to PHP 7.2 for the minimum, and the latest PHP 8 version, for the maximum.

See: D25537: Update PHP versions in install guide: Remove 5, Add 8..