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 Tasks: T15190

Differential Revision: https://we.phorge.it/D25108