With Haxe 4, the --php-lib phpLibFolder became -D php-lib phpLibFolder, in which case use can just use Compiler.define to set the variable. It’s quite possible that with Haxe 3, it will have the same effect, but you’ll have to try
But at the moment (HAXE 3.4.7) it doesn’t seem to be possible.
For the time being I will have a work-around. I simply check, if the conditions, set in the .hxml file, match the concept and have an error thrown, if this is not the case.
So thanks for your reply. It helped a lot.
And I am looking forward for the final release of HAXE 4.0.0.