PHP: rfc:allow-stringable-string-param-strict-mode

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====== PHP RFC: Allow Stringable For String Parameters in Strict Mode ======

  * Version: 1.0
  * Date: 2026-01-26
  * Author: Alexandre Daubois <alexandredaubois@php.net>, Nicolas Grekas <nicolasgrekas@php.net>
  * Status: Under discussion
  * Target Version: PHP 8.6
  * Implementation: TBD

===== Summary =====

This RFC proposes to accept <php>Stringable</php> objects for <php>string</php> type declarations in strict typing mode.

Currently, strict mode (<php>declare(strict_types=1)</php>) rejects <php>Stringable</php> values for <php>string</php> parameters, even though they have a well-defined string representation through the <php>__toString()</php> method.

===== Introduction =====

In strict typing mode, passing a <php>Stringable</php> object to a <php>string</php> parameter throws <php>TypeError</php>. In the default typing mode, the same call is accepted and the object is converted via <php>__toString()</php>.

<PHP>
<?php
declare(strict_types=1);

function process(string $value): void { }

$obj = new class implements Stringable {
    public function __toString(): string { return "hello"; }
};

process($obj);  // TypeError (works in non-strict mode)
?>
</PHP>

This inconsistency between typing modes requires workarounds despite <php>Stringable</php> being an **explicit contract** for safe string conversion.

===== Motivation =====

The <php>Stringable</php> interface was introduced in PHP 8.0 as an explicit contract indicating that an object can be safely converted to a string. However, strict typing mode still rejects these objects for <php>string</php> parameters.

This results in several issues:

  * **Inconsistent behavior**: The same code behaves differently between strict and non-strict modes for a well-defined conversion
  * **Verbose workarounds**: Developers must use patterns like <php>string|Stringable</php> unions with manual casting, or require callers to cast with <php>(string) $value</php>

Accepting <php>Stringable</php> in strict mode for <php>string</php> parameters is a natural relaxation that:

  * Respects the explicit <php>Stringable</php> contract
  * Reduces special cases in conversion behavior
  * Aligns with the longer-term direction of reducing the gap between typing modes (see the [[https://github.com/Girgias/unify-typing-modes-rfc|Unify PHP's typing modes]] gist)
  * Does not compromise type safety, as the conversion is well-defined and guaranteed by the interface

===== Proposal =====

Strict mode will accept <php>Stringable</php> objects for <php>string</php> parameters, converting them via <php>__toString()</php>:

<PHP>
<?php

declare(strict_types=1);

function greet(string $name): string { return "Hello, $name!"; }

$user = new class implements Stringable {
    public function __toString(): string { return "John"; }
};

greet($user);  // PHP 8.5: TypeError, PHP 8.6: "Hello, John!"
?>
</PHP>

===== Backward Compatibility Impact =====

This is a **relaxation** of type checking, not a breaking change. Code that currently works will continue to work.

The only potential compatibility issue is code that explicitly relies on strict mode rejecting <php>Stringable</php> objects. Most code receiving a <php>string</php> parameter treats it as a string value and does not care whether it originated from a <php>Stringable</php> object.

===== Proposed PHP Version =====

Next minor (PHP 8.6)

===== RFC Impact =====

Projects using strict types are expected to benefit from reduced verbosity and more consistent behavior. This rule aligns with the existing non-strict mode behavior, where <php>Stringable</php> is already accepted for <php>string</php> parameters.

===== Vote =====

Single yes/no vote requiring 2/3 majority.

<doodle title="Allow Stringable in strict mode in PHP 8.6?" voteType="single" closed="false">
   * Yes
   * No
   * Abstain
</doodle>

Voting opens [TBD] and closes [TBD].

===== References =====

  * Stringable RFC: https://wiki.php.net/rfc/stringable
  * PHP Type Declarations: https://www.php.net/manual/en/language.types.declarations.php
  * Unify PHP's typing modes (context/motivation): https://github.com/Girgias/unify-typing-modes-rfc
  * Discussion thread: TBD
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