diff --git a/.github/workflows/continuous-integration.yml b/.github/workflows/continuous-integration.yml
index 498a7ca7..bb318e00 100644
--- a/.github/workflows/continuous-integration.yml
+++ b/.github/workflows/continuous-integration.yml
@@ -14,7 +14,7 @@ jobs:
     name: "PHPUnit"
     uses: "doctrine/.github/.github/workflows/continuous-integration.yml@12.1.0"
     with:
-      php-versions: '["8.1", "8.2", "8.3", "8.4", "8.5"]'
+      php-versions: '["8.4", "8.5"]'
       phpunit-options-lowest: "--do-not-fail-on-deprecation"
     secrets:
       CODECOV_TOKEN: "${{ secrets.CODECOV_TOKEN }}"
diff --git a/UPGRADE.md b/UPGRADE.md
index ca89d1bc..07017c9d 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -6,6 +6,14 @@ awareness about deprecated code.
 - Use of our low-overhead runtime deprecation API, details:
   https://github.com/doctrine/deprecations/
 
+# Upgrade to 5.0
+
+## BC Break: added type declarations to constants
+
+The code base now has constants with type declarations. If you extend types
+from the library and override the constants, you will need to add compatible
+type declarations.
+
 # Upgrade to 4.2
 
 ## Add `getFieldValue` and `setFieldValue` to `ClassMetadata` implementation
diff --git a/composer.json b/composer.json
index 6ec6ca1d..4d1e4f3c 100644
--- a/composer.json
+++ b/composer.json
@@ -20,7 +20,7 @@
         {"name": "Marco Pivetta", "email": "ocramius@gmail.com"}
     ],
     "require": {
-        "php": "^8.1",
+        "php": "^8.4",
         "doctrine/deprecations": "^1",
         "doctrine/event-manager": "^1 || ^2",
         "psr/cache": "^1.0 || ^2.0 || ^3.0"
diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index f248bbd8..97f499fd 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -12,7 +12,7 @@
     <!-- Ignore warnings and show progress of the run -->
     <arg value="nps"/>
 
-    <config name="php_version" value="80100"/>
+    <config name="php_version" value="80400"/>
 
     <file>src</file>
     <file>tests</file>
diff --git a/phpstan.neon b/phpstan.neon
index 8a4291b4..6f358e4b 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -6,7 +6,7 @@ includes:
 
 parameters:
     level: 7
-    phpVersion: 80100
+    phpVersion: 80400
 
     paths:
         - src
diff --git a/src/Mapping/ClassMetadata.php b/src/Mapping/ClassMetadata.php
index 3ca89075..4cca90c9 100644
--- a/src/Mapping/ClassMetadata.php
+++ b/src/Mapping/ClassMetadata.php
@@ -35,7 +35,7 @@ public function getIdentifier(): array;
     /**
      * Gets the ReflectionClass instance for this mapped class.
      *
-     * @return ReflectionClass<T>
+     * @return ReflectionClass<covariant T>
      */
     public function getReflectionClass(): ReflectionClass;
 
diff --git a/src/Proxy.php b/src/Proxy.php
index 7394111f..77c4563d 100644
--- a/src/Proxy.php
+++ b/src/Proxy.php
@@ -14,12 +14,12 @@ interface Proxy
     /**
      * Marker for Proxy class names.
      */
-    public const MARKER = '__CG__';
+    public const string MARKER = '__CG__';
 
     /**
      * Length of the proxy marker.
      */
-    public const MARKER_LENGTH = 6;
+    public const int MARKER_LENGTH = 6;
 
     /**
      * Initializes this proxy if its not yet initialized.
diff --git a/tests/RuntimeReflectionPropertyTest.php b/tests/RuntimeReflectionPropertyTest.php
index f88b546b..eeead9f2 100644
--- a/tests/RuntimeReflectionPropertyTest.php
+++ b/tests/RuntimeReflectionPropertyTest.php
@@ -54,6 +54,7 @@ public function testGetValueOnProxyProperty(string $proxyClass): void
         $reflProperty = new RuntimeReflectionProperty($proxyClass, 'checkedProperty');
 
         self::assertSame('testValue', $reflProperty->getValue($mockProxy));
+        /** @phpstan-ignore unset.possiblyHookedProperty */
         unset($mockProxy->checkedProperty);
         self::assertNull($reflProperty->getValue($mockProxy));
     }