Part of Testing is the static analysis that your IDE can perform, or that standalone analysis tools can perform.
Some people don't like static analysis. Frankly, I think it's because those people don't know computer '''science''' and don't want to or can't grasp topics like wp:Reification (computer science).
Brent Roose of PHP Annotated discusses Static Analysis in this video about Generics in PHP
Static Analysis tools for PHP
Phan
see Phan
PHPStan
This is my favorite tool. See PHPStan