BooleanInBooleanAndRule.php

<?php declare(strict_types = 1);

namespace Slevomat\PHPStan\Rules;

class BooleanInBooleanAndRule implements \PHPStan\Rules\Rule
{

	public function getNodeType(): string
	{
		return \PhpParser\Node\Expr\BinaryOp\BooleanAnd::class;
	}

	/**
	 * @param \PhpParser\Node\Expr\BinaryOp\BooleanAnd $node
	 * @param \PHPStan\Analyser\Scope $scope
	 * @return string[] errors
	 */
	public function processNode(\PhpParser\Node $node, \PHPStan\Analyser\Scope $scope): array
	{
		$leftType = $scope->getType($node->left);
		if (!BooleanRuleHelper::passesAsBoolean($leftType)) {
			return [
				sprintf(
					'Only booleans are allowed in &&, %s given on the left side.',
					$leftType->describe()
				),
			];
		}

		$rightType = $scope->getType($node->right);
		if (!BooleanRuleHelper::passesAsBoolean($rightType)) {
			return [
				sprintf(
					'Only booleans are allowed in &&, %s given on the right side.',
					$rightType->describe()
				),
			];
		}

		return [];
	}

}

BooleanRuleHelper.php

<?php declare(strict_types = 1);

namespace Slevomat\PHPStan\Rules;

class BooleanRuleHelper
{

	public static function passesAsBoolean(\PHPStan\Type\Type $type): bool
	{
		if ($type instanceof \PHPStan\Type\BooleanType) {
			return true;
		}
		if (
			$type instanceof \PHPStan\Type\MixedType
			&& !$type->isExplicitMixed()
		) {
			return true;
		}

		return false;
	}

}