PHP: Remove use of own cacerts.txt file

Minimal change actually needed.

From inside CUDN, both these work:

docker run --rm -ti -v $(pwd)/src/php:/src:ro php:5.6 php src/example.php
docker run --rm -ti -v $(pwd)/src/php:/src:ro php:7.4 php src/example.php

Closes: #5 (closed)