A PhpStan rule to ban changes on public members outside a class.

DontWriteToPublicPropertiesRule.php

<?php declare(strict_types=1);

use PhpParser\Node;
use PhpParser\Node\Expr;
use PhpParser\Node\Expr\Assign;
use PhpParser\Node\Expr\AssignOp;
use PhpParser\Node\Expr\PropertyFetch;
use PhpParser\Node\Expr\StaticPropertyFetch;
use PhpParser\Node\Identifier;
use PHPStan\Analyser\Scope;
use PHPStan\Rules\Properties\PropertyReflectionFinder;
use PHPStan\Rules\Rule;

use function iterator_to_array;

final class DontWriteToPublicPropertiesRule implements Rule
{
    public function __construct(PropertyReflectionFinder $propertyReflector)
    {
        $this->propertyReflector = $propertyReflector;
    }

    public function getNodeType(): string
    {
        return Expr::class;
    }

    public function processNode(Node $node, Scope $scope): array
    {
        return iterator_to_array($this->validate($node, $scope), false);
    }

    private function validate(Node $node, Scope $scope)
    {
        if (!$node instanceof Assign && !$node instanceof AssignOp) {
            return;
        }

        $property = $node->var;
        if (!$property instanceof PropertyFetch && !$property instanceof StaticPropertyFetch) {
            return;
        }

        if (!$property->name instanceof Identifier) {
            return;
        }

        $reflection = $this
            ->propertyReflector
            ->findPropertyReflectionFromNode(
                $property,
                $scope
            )
        ;

        if ($reflection === null) {
            return;
        }

        if ($scope->getClassReflection() === $reflection->getDeclaringClass()) {
            return;
        }

        yield sprintf(
            'Cannot write to public property "%s:%s".',
            $reflection->getDeclaringClass()->getName(),
            $property->name
        );
    }

    private $propertyReflector;
}