PHP 7.4 reached end of life on 28 November 2022. Laravel 10, the oldest release in the table above, needs PHP 8.1, so an application on 7.4 is also on an older framework.
PHP 8 changes behaviour as well as syntax. Comparing a number with a non-numeric string now casts the number to a string, so 0 == "foo" is false where it used to be true, and nothing warns you. each() and array_key_exists() on objects are gone, non-static methods can no longer be called statically, and the @ operator no longer hides fatal errors.
We add tests around the flows that matter before changing anything, then move PHP and the framework together. We aim for PHP 8.3 at least, because Laravel 13 requires it, and PHP 8.3 has security support until 31 December 2027, 8.4 until 31 December 2028 and 8.5 until 31 December 2029.