Implement phan stan

[phil-davis]

Description

• implement phan and phpstan in Makefile and drone pipeline steps
• add phan entries in the drone matrix
• switch coding-standard from PHP 7.2 to 5.6 (phan effectively does the lint/syntax job for PHP 7.*)
• code changes to fix issues reported by phan, including moving the locking folder code under lib

This currently includes the pending commits from PR #74 which also awaits review and merge. This PR has been rebased after merge of PR #74

Checklist:

• [x] Latest changes are published according to instructions in README.md

[phil-davis]

@patrickjahns any review comments? merge?