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PHP RFC: Deprecate boolean to string coercion

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://wiki.php.net/rfc/coercive_sth#changes_to_internal_functions