Fix static analysis errors in Data class method chaining

[h-marumoto]

This PR fixes #1143

This PR resolves static analysis errors when using method chaining.

The Smarty class extends the Data class, but the PHPDoc return types for methods in the Data class (assign(), append(), etc.) were documented as returning Data.

As a result, when using method chaining like the following, static analysis tools would interpret that a Data instance is returned instead of a Smarty instance, and would report errors that Smarty-specific methods (e.g., display()) do not exist.

(new Smarty())->assign("x", "x")->display('template.tpl');
// Undefined method 'display'.

I changed the PHPDoc return type from Data to $this. By using $this, static analysis tools can correctly recognize that an instance of the actually invoked class (in this case, Smarty) is returned.

This change only modifies PHPDoc comments and has no effect on runtime behavior.

[wisskid]

Hi @h-marumoto I see how this fix would work and would probably be fine for most cases, but @return $this doesn't seem to be official phpdoc syntax? In your defense: the codebase already contains 3 instances of @return $this in the Smarty\Smarty class.

[h-marumoto]

Hi @wisskid , sorry for the slow response!

Fair point — strictly speaking @return $this was never ratified.
It is, however, officially supported by phpDocumentor, and PHPStan, Psalm, and PhpStorm all recognize it — PHPStan even lists $this as a basic type in its PHPDoc parser, and Psalm internally treats it as a type token equivalent to static.
Ref: https://docs.phpdoc.org/guide/references/phpdoc/tags/return.html#the-this-keyword

I'd prefer to keep $this here since it's the most precise annotation for these methods (the same instance is returned, not just an instance of the called class), and it matches the existing usage in Smarty\Smarty.
Let me know if you'd like any changes!