please be careful.
It is correct that v17.x is not compatible to Catalina when included PHP is used.
Minimum Version for Catalina to use included PHP is 4D v17 R5, better the soon to be released R6, which includes some some other modifications for Catalina.
If you need to use v17, use v17.3 HF1, which includes some other modifications for Catalina, but not PHP. If you need to use PHP, use an external PHP interpreter (see documentation for external PHP interpreter).
Please be aware that v17.x used PHP 5, while 4D v17 R5 and newer is using PHP 7.
PHP 7 and 5 are not fully compatible, major code changes might be required.
Apple ships with Catalina PHP 7.(3.8), so script changes are also needed if you use system PHP.
Using the PHP interpreter from R5 with v17.x seems to work, but (beside that it is version 7 and not 5), is not tested and not developed for this usage, you might run into serious issues, so we cannot recommend it.