1 pointThanks for the question! I'm not terribly worried about this. Microsoft said they're not going to provide builds of PHP for Windows anymore. Certainly someone else will pick up the torch. Also, to me, this is more of an issue for people learning or developing on Windows. I don't have hard numbers but I assume the vast majority of servers using PHP are running Linux.
1 pointYes, both versions are comparable with respect to validating the gender. To use the NULL coalescing operator in Script 2.4, you'd probably just write it like your version of Script 2.3 (note that you don't need the $gender = NULL in there though).
1 pointI wouldn't say that line is necessary. The $gender variable is already assigned a value before being used so assigning it a "false" value has no impact.