# RFC: Deprecate boolean to string coercion

- Status: Inactive
- Authors: George Peter Banyard girgias@php.net, Ilija Tovilo tovilo.ilija@gmail.com
- Date: 2021-06-22
- Version: 0.1
- Target: PHP 8.1
- Implementation: https://github.com/php/php-src/pull/9999999
- Original: https://wiki.php.net/rfc/deprecate-boolean-string-coercion
- Page: https://externals.io/rfc/deprecate-boolean-string-coercion
- More for agents: search with https://externals.io/search.md?q={query} · RFCs at https://externals.io/rfc.md · JSON API at https://externals.io/api · documentation at https://externals.io/llms.txt

## Discussions on the mailing list

- [\[RFC\] Deprecate boolean to string coercion](https://externals.io/message/115048.md) — started on 2021-06-22, 4 messages

---

## Introduction

PHP is a dynamically typed language and as such implicit type coercion naturally arises, most of these are harmless and rather convenient. However, the conversion from boolean to string is asymmetric, as `false` gets coerced to an empty string `''` and `true` to the string `'1'`, and is a rare occurrence which most likely hints to a bug.

## Proposal

Emit an `E_DEPRECATED` deprecation diagnostic for implicit coercion of a boolean to a string.

The diagnostic message is: > Implicit bool to string coercion is deprecated

Raise this deprecation diagnostic to a TypeError in the next major version (PHP 9.0).

Amending the type signature of the following functions as their usage with a boolean argument is common:

- `ini_set()`

## Rationale

A code audit of the impact of this change was realized back in 2015 for the “Coercive Types for Function Arguments” RFC \[1\] and any occurrence of this happening was a symptom of a bug rather then desirable behaviour.

PHP's boolean conversion to string is also unique, databases will convert `true`/`false` into `'1'`/`'0'`, and other programming languages if they support such a conversion usually convert it to `'true'`/`'false'`. Moreover, the fact that `false` gets converted to an empty string `''` means that as of PHP 8.0, it does not produce a valid numeric string, resulting in a broken chain of implicit coercion if the string is used in a numeric context later on.

## Backward Incompatible Changes

The following operations will now emit an E\_DEPRECATED if a boolean is used:

- Concatenation operator `.`
- `echo` language construct
- `print` language construct
- String interpolation
- Assignment to a typed property of type `string` in coercive typing mode
- Argument for a parameter of type `string` for both internal and userland functions in coercive typing mode
- Returning such a value for userland functions declared with a return type of `string` in coercive typing mode

## Proposed PHP Version

Deprecation and function signature changes: next minor version, i.e. PHP 8.1.

Promotion to TypeError: next major version, i.e. PHP 9.0.

## Unaffected PHP Functionality

- Manual casting to string will not emit a deprecation diagnostic.
- String to boolean implicit coercions are not affected.
- Strict type behaviour is unaffected.
- Coercion from bool to int
- Coercion from bool to float
- `printf()` family of functions

## Future scope

- Deprecating casting boolean to strings
- Changing the output of `false` being casted to string

## Proposed Voting Choices

As per the voting RFC a yes/no vote with a 2/3 majority is needed for this proposal to be accepted.

## Patches and Tests

Links to any external patches and tests go here.

If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed.

Make it clear if the patch is intended to be the final patch, or is just a prototype.

For changes affecting the core language, you should also provide a patch for the language specification.

## Implementation

After the project is implemented, this section should contain

- the version(s) it was merged into
- a link to the git commit(s)
- a link to the PHP manual entry for the feature
- a link to the language specification section (if any)

## References

\[1\]: [PHP RFC: Coercive Types for Function Arguments](https://externals.io/rfc/coercive_sth)