Common mistakes to avoid in PHP
List of Stack Overflow questions about mistakes to avoid in PHP
(The only proper) [PHP] PDO tutorial
A nice tutorial on PDO, the built-in SQL interface for PHP.
In particular, the official documentation doesn't say much about querying, but it's covered well here.
The other pages on the site are good too.
Psalm PHP Static Analyzer
A static analyzer with (optionally very strict) type checking. Developed at Vimeo.
Includes a language server (link to docs)