diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index f26e88c..5717157 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -30,3 +30,16 @@ variables:
   _PHPUNIT_CONCURRENT: 0
   _CSPELL_WORDS: 'cloudflared, cleanstring, ctools, dpagini, ffpu, multiext, oleh, origname, pagini, vehera, voleger, тест'
   OPT_IN_TEST_PREVIOUS_MAJOR: '1'
+
+# The module requires PHP 8.2. Test Drupal 10 on PHP 8.2, not on 8.1.
+# CORE_PREVIOUS_PHP_MIN is a CI/CD settings variable. A settings variable
+# wins over any value in this file, so set the PHP version on the job.
+composer (previous major):
+  variables:
+    PHP_VERSION: '8.2'
+
+# Drupal 10's test bootstrap changes directory into web/. PHPUnit 9 then
+# reads the configuration file again from there, so a relative path fails.
+phpunit (previous major):
+  variables:
+    _PHPUNIT_EXTRA: '--no-coverage --configuration=$CI_PROJECT_DIR/phpunit.gitlab-ci.xml'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d1a7c2a..240abf7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,16 @@ This file records the changes in each release of File (Field) Paths.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 Releases follow the Drupal.org `8.x-1.x` contrib versioning scheme.
 
+## Unreleased
+
+### Changed
+
+- [#3622262](https://www.drupal.org/i/3622262): Declared PHP 8.2 as the
+  minimum version. `8.x-1.0-rc2` already needed it, because it declares five
+  classes `readonly`, but it did not say so and failed with a parse error on
+  PHP 8.1. Composer and Drupal now refuse PHP 8.1. Sites on PHP 8.1 should
+  pin `8.x-1.0-rc1`, because Composer there still resolves to `8.x-1.0-rc2`.
+
 ## 8.x-1.0-rc2 - 2026-09-07
 
 ### Added
diff --git a/README.md b/README.md
index 233b360..087a36a 100644
--- a/README.md
+++ b/README.md
@@ -31,7 +31,7 @@ Submit bug reports and feature suggestions, or track changes in the
 
 ## Requirements
 
-This module requires the following modules:
+This module requires PHP 8.2 or later and the following modules:
 
 - [Pathauto](https://www.drupal.org/project/pathauto)
 - [Redirect](https://www.drupal.org/project/redirect)
diff --git a/composer.json b/composer.json
index 3580524..4dc3f6d 100644
--- a/composer.json
+++ b/composer.json
@@ -4,6 +4,9 @@
     "keywords": ["file", "field", "path", "token", "drupal"],
     "type": "drupal-module",
     "license": "GPL-2.0-or-later",
+    "require": {
+        "php": ">=8.2"
+    },
     "require-dev": {
         "drupal/ctools": "*",
         "drupal/pathauto": "*",
diff --git a/filefield_paths.info.yml b/filefield_paths.info.yml
index 442ac99..56d7829 100644
--- a/filefield_paths.info.yml
+++ b/filefield_paths.info.yml
@@ -3,6 +3,7 @@ description: 'Adds improved Token based file sorting and renaming functionalitie
 package: Fields
 configure: filefield_paths.admin_settings
 core_version_requirement: ^10.3 || ^11
+php: 8.2
 type: module
 dependencies:
   - drupal:file
diff --git a/tests/src/Unit/PhpRequirementTest.php b/tests/src/Unit/PhpRequirementTest.php
new file mode 100644
index 0000000..1f21c26
--- /dev/null
+++ b/tests/src/Unit/PhpRequirementTest.php
@@ -0,0 +1,175 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\filefield_paths\Unit;
+
+use PHPUnit\Framework\Attributes\Group;
+use Composer\Semver\Semver;
+use Drupal\Component\Serialization\Yaml;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests that the module declares the PHP version its code needs.
+ *
+ * The code uses syntax that an older PHP cannot parse. Composer and Drupal
+ * refuse that PHP only if the module declares its minimum version.
+ *
+ * @group filefield_paths
+ * @coversNothing
+ */
+#[Group('filefield_paths')]
+class PhpRequirementTest extends UnitTestCase {
+
+  /**
+   * The minimum PHP version for a readonly class.
+   */
+  private const READONLY_CLASS_PHP = '8.2';
+
+  /**
+   * The minimum PHP version for Drupal 10, the oldest supported core.
+   */
+  private const DRUPAL_10_PHP = '8.1';
+
+  /**
+   * Tests that the info file declares the PHP version the code needs.
+   */
+  public function testInfoFileDeclaresRequiredPhp(): void {
+    $php = $this->infoFilePhp();
+    $required = $this->requiredPhp();
+
+    // Drupal compares this value with version_compare() too.
+    $this->assertTrue(
+      version_compare($php, $required, '>='),
+      sprintf('filefield_paths.info.yml declares PHP %s, but the code needs PHP %s.', $php, $required),
+    );
+  }
+
+  /**
+   * Tests that composer.json rejects a PHP version the code cannot run on.
+   */
+  public function testComposerRejectsOlderPhp(): void {
+    $constraint = $this->composerPhp();
+    $required = $this->requiredPhp();
+    $below = $this->versionBelow($required);
+
+    $this->assertFalse(
+      Semver::satisfies($below, $constraint),
+      sprintf('composer.json allows PHP %s with "%s", but the code needs PHP %s.', $below, $constraint, $required),
+    );
+  }
+
+  /**
+   * Tests that composer.json and the info file declare the same minimum.
+   */
+  public function testComposerMatchesInfoFile(): void {
+    $constraint = $this->composerPhp();
+    $php = $this->infoFilePhp();
+
+    $this->assertTrue(
+      Semver::satisfies($php . '.0', $constraint),
+      sprintf('composer.json "%s" rejects PHP %s, the minimum in the info file.', $constraint, $php),
+    );
+    $this->assertFalse(
+      Semver::satisfies($this->versionBelow($php), $constraint),
+      sprintf('composer.json "%s" allows a PHP older than %s, the minimum in the info file.', $constraint, $php),
+    );
+  }
+
+  /**
+   * Returns the minimum PHP version in the info file.
+   *
+   * @return string
+   *   The value of the php key, for example "8.2".
+   */
+  private function infoFilePhp(): string {
+    $info = Yaml::decode($this->readModuleFile('filefield_paths.info.yml'));
+    $this->assertIsArray($info);
+    $php = $info['php'] ?? NULL;
+    $this->assertIsScalar($php, 'filefield_paths.info.yml does not declare a minimum PHP version.');
+
+    return (string) $php;
+  }
+
+  /**
+   * Returns the PHP constraint in composer.json.
+   *
+   * @return string
+   *   The constraint, for example ">=8.2".
+   */
+  private function composerPhp(): string {
+    $composer = json_decode($this->readModuleFile('composer.json'), TRUE);
+    $this->assertIsArray($composer);
+    $require = $composer['require'] ?? [];
+    $this->assertIsArray($require);
+    $constraint = $require['php'] ?? NULL;
+    $this->assertIsString($constraint, 'composer.json does not require a minimum PHP version.');
+
+    return $constraint;
+  }
+
+  /**
+   * Returns the minimum PHP version that the code in src/ needs.
+   *
+   * @return string
+   *   The PHP version, for example "8.2".
+   */
+  private function requiredPhp(): string {
+    $files = new \RecursiveIteratorIterator(
+      new \RecursiveDirectoryIterator($this->moduleRoot() . '/src', \FilesystemIterator::SKIP_DOTS),
+    );
+    foreach ($files as $file) {
+      if (!$file instanceof \SplFileInfo || $file->getExtension() !== 'php') {
+        continue;
+      }
+      $code = (string) file_get_contents($file->getPathname());
+      if (preg_match('/^\s*(?:(?:final|abstract)\s+)*readonly\s+(?:(?:final|abstract)\s+)*class\s/m', $code) === 1) {
+        return self::READONLY_CLASS_PHP;
+      }
+    }
+
+    return self::DRUPAL_10_PHP;
+  }
+
+  /**
+   * Returns the last patch release before a minor version.
+   *
+   * @param string $version
+   *   A minor version, for example "8.2".
+   *
+   * @return string
+   *   The release before it, for example "8.1.99".
+   */
+  private function versionBelow(string $version): string {
+    [$major, $minor] = array_map(intval(...), explode('.', $version));
+
+    return $minor > 0 ? sprintf('%d.%d.99', $major, $minor - 1) : sprintf('%d.99.99', $major - 1);
+  }
+
+  /**
+   * Reads a file from the module root.
+   *
+   * @param string $name
+   *   The file name, relative to the module root.
+   *
+   * @return string
+   *   The file contents.
+   */
+  private function readModuleFile(string $name): string {
+    $contents = file_get_contents($this->moduleRoot() . '/' . $name);
+    $this->assertIsString($contents, sprintf('Cannot read %s.', $name));
+
+    return $contents;
+  }
+
+  /**
+   * Returns the module root directory.
+   *
+   * @return string
+   *   The absolute path of the directory that holds the info file.
+   */
+  private function moduleRoot(): string {
+    return dirname(__DIR__, 3);
+  }
+
+}