PHP 8.5 behavior change: header() removes headers by name prefix

[Artemeey]

Description

Fast example

php 8.4: https://php-play.dev/?c=DwfgDgFmBQ0QpgQwCbwE4AoDkiBcACARiwEoBuOJVTHAWgBd4Bneg48yldbPI0i6ADdEaAPrIArgFswGBFzRNRAGwCWLDCQ5A&v=8.4&f=console

php 8.5: https://php-play.dev/?c=DwfgDgFmBQ0QpgQwCbwE4AoDkiBcACARiwEoBuOJVTHAWgBd4Bneg48yldbPI0i6ADdEaAPrIArgFswGBFzRNRAGwCWLDCQ5A&v=8.5&f=console

Reproduction script

<?php

header('a: 1');
header('a-test: 1');
header('a: 1');

var_dump(headers_list());

Output (PHP 8.4.7, correct)

array(3) {
  [0]=>
  string(28) "X-Powered-By: PHP/8.4.17-dev"
  [1]=>
  string(9) "a-test: 1"
  [2]=>
  string(4) "a: 1"
}

Output (PHP 8.5.2, broken)

array(2) {
  [0]=>
  string(27) "X-Powered-By: PHP/8.5.2-dev"
  [1]=>
  string(4) "a: 1"
}

Problem statement

In PHP 8.5.x header() appears to remove previously set headers whose names share a prefix with the header being replaced. This differs from PHP 8.4.x behavior.

In the example below, replacing a (calling header('a: 1') twice) also removes a-test, despite a-test being a different header name.

Expected behavior

Replacing a header should affect only headers with the exact same name (a), not other headers (a-test) with a shared prefix.

Actual behavior

Replacing <base> also removes <base>-<suffix> headers.

Question

Is this an intended change in PHP 8.5 header handling?

PHP Version

PHP 8.4.7 (cli) (built: May 14 2025 18:01:53) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.4.7, Copyright (c) Zend Technologies

PHP 8.5.2 (cli) (built: Jan 21 2026 10:06:03) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.5.2, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.2, Copyright (c), by Zend Technologies

Operating System

Ubuntu 24.04

[iluuu1994]

Bisected to 18dee43. /cc @NattyNarwhal @bukka

[NattyNarwhal]

Yes, this is a regression. I'll take a look at this.

[NattyNarwhal]

GH-21020 ready, let me know if it works for you.

[isaumya]

Hi,
Thanks for reporting this. I've been facing this issue making me not use 8.5. Any idea on which version of 8.5 this header issue is getting fixed? It would be great help to know that.

[NattyNarwhal]

8.5.3 was tagged before this fix was merged, so expect it in 8.5.4.

[isaumya]

Hi @NattyNarwhal,
Can you please confirm if it has been added in PHP 8.5.4? I do not see any reference to GH-21020 in the changelog