Catch use of $$this

[ancarda]

Feature request

A colleague of mine caught a bug that used $$this->methodCall(). I know the phpstan/phpstan-strict-rules plugin can ban the use of $$, however I think PHPStan, out of the box, should prohibit $$this. Given $this always refers to an instance of a class, and not a string, it can't possibly be used as the lookup for something on the symbol table.

I played around with this in 3v4l.org, $$this just produces NULL, which PHPStan (0.12.81) does not pickup on to stop the method call. Consider the following code (available at https://3v4l.org/mhr52):

<?php

declare(strict_types=1);

final class Greeter
{
    public function greet(): void
    {
        echo $$this->greetText();
    }

    public function greetText(): string
    {
        return 'Hello World';
    }
}

$g = new Greeter();
$g->greet();

You will get the following PHP error when trying to run:

Fatal error: Uncaught Error: Object of class Greeter could not be converted to string in /in/mhr52:9

However, PHPStan (with --level=max) will not report any issues.

[ancarda]

After further experimentation, it seems this would work if you have __toString, so perhaps PHPStan could check for that method's existence, and if it's not defined, reject $$this?

[ondrejmirtes]

Hi, this is very easy to implement.

• Have a rule that listens to Variable AST node
• Get the $scope->getType($variable->name) when $variable->name is Expr
• If $type->toString() is ErrorType, report the error.

[ancarda]

Thank you @ondrejmirtes! I'll give it a go tonight. I'd love to be able to finally contribute something to PHPStan!

[ondrejmirtes]

There's also the opportunity to catch the same problem with $foo->{$method}() etc.