Use the latest coding standard, at the moment of writing this is PSR-12. Congirure your IDE to check this standard while developing. You can use PHP_CodeSniffer.
Make sure your application can be tested automatically.
Use declare(strict_types = 1);
at the top of every PHP file. Strict typing
Use return types for methods
public function foo(): string
Use strong typed parameters in methods
public function bar(string $foo, int $baz)
Try to use only CRUD methods in controllers. When using only CRUD methods the controller can't be bloated by to many methods. Bloaters
Try to build as small as possible methods. Long method
Try to build as small as possible classes. Big classes can 'wear to many hats'. Large class
Try to use as little as possible parameters in methods. Long parameter list