Operator Type Specifying Extensions

If you’re working with PHP extensions that overload arithmetic operators (like GMP or BCMath), you can write an operator type specifying extension to describe how operators like +, -, *, /, **, %, ^ should infer types when applied to your custom objects.

The implementation is all about applying the core concepts so check out that guide first and then continue here.

This is the interface your extension needs to implement:

namespace PHPStan\Type;

interface OperatorTypeSpecifyingExtension
{

	public function isOperatorSupported(string $operatorSigil, Type $leftSide, Type $rightSide): bool;

	public function specifyType(string $operatorSigil, Type $leftSide, Type $rightSide): Type;

}

The extension has two methods:

• isOperatorSupported() — receives the operator sigil (e.g. +, -, *) and the types of both operands. Return true when your extension wants to handle this combination.
• specifyType() — called when isOperatorSupported() returns true. Return the resulting type of the operation.

Here’s an example for a Decimal class that supports arithmetic:

namespace App\PHPStan;

use PHPStan\Type\ObjectType;
use PHPStan\Type\OperatorTypeSpecifyingExtension;
use PHPStan\Type\Type;

class DecimalOperatorExtension implements OperatorTypeSpecifyingExtension
{

	public function isOperatorSupported(string $operatorSigil, Type $leftSide, Type $rightSide): bool
	{
		$decimalType = new ObjectType(\App\Decimal::class);

		if (in_array($operatorSigil, ['+', '-', '*', '/'], true)) {
			return $decimalType->isSuperTypeOf($leftSide)->yes()
				&& $decimalType->isSuperTypeOf($rightSide)->yes();
		}

		return false;
	}

	public function specifyType(string $operatorSigil, Type $leftSide, Type $rightSide): Type
	{
		return new ObjectType(\App\Decimal::class);
	}

}

With this extension, PHPStan knows that arithmetic on Decimal objects produces a Decimal:

function calculate(Decimal $a, Decimal $b): void
{
	$c = $a + $b;
	// Without extension: mixed (PHPStan doesn't know about operator overloading)
	// With extension: Decimal
	\PHPStan\dumpType($c); // App\Decimal
}

The implementation needs to be registered in your configuration file:

services:
	-
		class: App\PHPStan\DecimalOperatorExtension
		tags:
			- phpstan.broker.operatorTypeSpecifyingExtension