everything has to be upgraded. Older version of composer doesn't work with php 7.3, cakephp must be carefully upgraded. In my tests we even got different query results.
So maybe upgrading PHP itself might not be that bad, something I can't tell you for sure, but in my experience nobody uses vanilla PHP, they all wrap it in frameworks, and those must all be carefully upgraded too.
So maybe upgrading PHP itself might not be that bad, something I can't tell you for sure, but in my experience nobody uses vanilla PHP, they all wrap it in frameworks, and those must all be carefully upgraded too.