diff --git a/composer.json b/composer.json
index aba213db28..86e6d05780 100644
--- a/composer.json
+++ b/composer.json
@@ -3,10 +3,16 @@
   "description": "PHPUnit dependencies for the PHP shipped in roles/*/files/php/.",
   "type": "project",
   "license": "MIT",
+  "require": {
+    "php": ">=8.3"
+  },
   "require-dev": {
-    "phpunit/phpunit": "^13.3"
+    "phpunit/phpunit": "^12.0"
   },
   "config": {
+    "platform": {
+      "php": "8.3"
+    },
     "sort-packages": true
   },
   "scripts": {
diff --git a/utils/install/php.py b/utils/install/php.py
index f46a1df4b2..c7fdd71147 100644
--- a/utils/install/php.py
+++ b/utils/install/php.py
@@ -19,7 +19,7 @@ _PHPUNIT_EXTENSIONS = ("dom", "mbstring", "xmlwriter")
 def ensure_php_toolchain() -> None:
     """Install the PHP interpreter, Composer and the extensions PHPUnit needs.
 
-    ``phpunit/phpunit ^13`` declares ``_PHPUNIT_EXTENSIONS`` as platform
+    ``phpunit/phpunit`` declares ``_PHPUNIT_EXTENSIONS`` as platform
     requirements and Composer refuses to resolve the vendor tree without them,
     so they are part of the toolchain rather than of the suite consuming it.
     A distro ships them as packages separate from the interpreter, which is why