rP55596bfad523

PHP Pitfalls: mention strlen() deprecation since PHP 8.1

Summary:
- expand documentation about PHP Pitfalls to mention strlen() in PHP 8.1
- mention phutil_string_cast()
- mention phutil_nonempty_string()
- add a commodity link from PHP Contributors Manual

Ref T15190

Test Plan: * check with your big eyes for typos

Reviewers: O1 Blessed Committers, Cigaryno, avivey

Reviewed By: O1 Blessed Committers, Cigaryno, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Tags: #documentation

Maniphest…