Deprecate Undefined Constant Usage in php.ini Files (GH-20060)

From: Date: Sun, 05 Oct 2025 12:54:59 +0000
Subject: Deprecate Undefined Constant Usage in php.ini Files (GH-20060)
References: 1  Groups: php.internals 
Request: Send a blank email to internals+get-128781@lists.php.net to get a copy of this message 
Hello list,
I'd like to propose a change in PHP behaviour and would like to request
your comments.

Find my proposals' draft below.

Thank you in advance for your attention and thoughts, as usual I can
benefit from your help. If you have voting power and would consider
to vote on the topic, your rejection or acceptance rationale
appreciated. I would like to draft an implementation and could
incorporate the feedback then beforehand already.

Best,

-- hakre


## Deprecate Undefined Constant Usage in php.ini Files

The php.ini file parser (as used by parse_ini_file()) supports
constant resolution, including core predefined constants. However, the
current behavior with undefined constants is inconsistent with PHP 8's
strict handling of undefined constants in regular PHP code.

### Problem Statement

#### Current Inconsistent Behavior

In PHP 8 scripts, undefined constants trigger immediate fatal errors:


    <?php
    $var = X_ERROR; // Fatal error: Undefined constant "X_ERROR"


However, in php.ini files, undefined constants are silently handled
using PHP 4-era semantics:


    [PHP]
    error_reporting = X_ERROR|E_WARNING|E_PARSE|E_NOTICE


This silently:

1. Converts X_ERROR to string "X_ERROR"
2. Converts that string to integer 0 in bitwise context
3. Results in value 14 (0 | 4 | 2 | 8) without any indication of the
   error

This creates invisible configuration errors that are difficult to debug.

### Proposed Change

#### Proposal: Add Diagnostics for Undefined Constants

We propose emitting diagnostics when undefined constants are encountered
in configuration files:

**During startup (php.ini parsing):**



    PHP: Use of undefined constant NAME_OF_CONSTANT - assumed 'NAME_OF_CONSTANT' (this will throw an Error in a future version of PHP)



**During runtime (parse_ini_file/string):**



    PHP Warning: Use of undefined constant NAME_OF_CONSTANT - assumed 'NAME_OF_CONSTANT' (this will throw an Error in a future version of PHP) in filename on line X



This makes configuration errors visible and paves the way for future
strict handling.

### Message Severity Discussion

The proposed warning message and severity mirror the PHP 7.2 approach
for undefined constants in scripts:



    Warning: Use of undefined constant UNDEFINED_CONSTANT - assumed 'UNDEFINED_CONSTANT' (this will throw an Error in a future version of PHP)



Alternative severity levels could be considered:


| Severity     | Wording      | Intent              |

|--------------|--------------|---------------------|
| E_DEPRECATED | "may throw"  | Softer transition   |

| E_WARNING    | "will throw" | Clear future intent |
| E_NOTICE     | "will throw" | Less intrusive      |


The PHP 7.2 precedent suggests E_WARNING is appropriate for this level
of issue.

### Benefits

- Early detection: Configuration errors become visible immediately
- Consistency: Aligns ini parsing behavior with PHP 8 language
  semantics
- Fail-fast: Helps users identify and fix configuration mistakes sooner
- Future-proof: Enables eventual strict mode matching PHP script
  behavior

### Implementation Notes

The change would be implemented in zend_ini_get_constant() by adding
diagnostics when constant resolution fails. The current ini_error()
function uses ZEND_COLD (compiler hint for infrequently executed
code), which remains appropriate since undefined constants should be
rare in production configurations.

This infrastructure would also support a future transition to throwing
Errors, though the error handling mechanism might need adjustment at
that time.

* Note: This section is preliminary and subject to implementation

*       details.


### References

#### PHP Manual Pages

a.) parse_ini_file() <https://www.php.net/manual/en/function.parse-ini-file.php>
b.) The Configuration File <https://www.php.net/manual/en/configuration.file.php>
c.) Core Predefined Constants <https://www.php.net/manual/en/reserved.constants.php#reserved.constants.core>


#### 3v4l.org PHP Codes

d.) https://3v4l.org/7PA3u Undefined constant
e.) https://3v4l.org/vC6mg Unsupported operand types

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