php displays code partly normal and partly as text

I've installed a debian 10 with ispconfig, php 5.6 and 5.7 (thanks for the great tutorials).
I've an application (15 years old), I was using with php 5.5.45 on a debian 7.
I'd like to move it to debian 10.
It doesn't matter if I'using fpm or fast-cgi with php 5.6.
The result is the same : part of the code is displayed correctly, part is displayed as text (cf the attached image).
Many thanks in advance for your help.
BRgds.
Michel

 

Many thanks for your help. It works,