PHP: rfc:deprecate-boolean-string-coercion

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====== PHP RFC: Deprecate boolean to string coercion ======

  * Version: 0.1
  * Date: 2021-06-22
  * Author: George Peter Banyard <girgias@php.net>, Ilija Tovilo <tovilo.ilija@gmail.com>
  * Status: Under Discussion
  * Target Version: PHP 8.1
  * Implementation: [[https://github.com/php/php-src/pull/9999999|https://github.com/php/php-src/pull/9999999]]
  * First Published at: [[http://wiki.php.net/rfc/deprecate-boolean-string-coercion|http://wiki.php.net/rfc/deprecate-boolean-string-coercion]]
  * GitHub mirror: [[https://github.com/Girgias/php-rfc-bool-string-deprecation|https://github.com/Girgias/php-rfc-bool-string-deprecation]]

===== 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:

  * <php>ini_set()</php>

===== 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 ''%%.%%''
  * <php>echo</php> language construct
  * <php>print</php> 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
  * <php>printf()</php> 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]: [[rfc:coercive_sth|PHP RFC: Coercive Types for Function Arguments]]
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