This is a suggestion to establish a new rule: "Every file with .php extension must have correct PHP syntax (in other words, it must pass php -l). If there is a reason (eg for unit tests) to have a PHP file with broken syntax, it must have different extension."

Such a rule would be enforced by a new pre-release step in the integration chain. Something like the following shell in Jenkins:

#!/bin/bash -e

find -L ${pathtomoodle} -name 'index.php' -print0 | xargs -0 -n 1 -P 4 php -l

It ends with return status 0 (success) if no syntax error is detected. It stops with the return status >0 otherwise and Jenkins marks such a build as a failure.