diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index ba35236..d2616fb 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         operating-system: ['ubuntu-latest']
-        php-version: ['8.3', '8.4']
+        php-version: ['8.1', '8.2', '8.3', '8.4']
 
     name: PHP ${{ matrix.php-version }} Test on ${{ matrix.operating-system }}
     steps:
diff --git a/composer.json b/composer.json
index 6bea8e5..722222f 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,7 @@
         }
     ],
     "require": {
-        "php": ">=8.3"
+        "php": ">=8.1"
     },
     "require-dev": {
         "phpunit/phpunit": "10.x"
diff --git a/src/Manifest.php b/src/Manifest.php
index c022f50..1b7ec17 100644
--- a/src/Manifest.php
+++ b/src/Manifest.php
@@ -5,7 +5,7 @@
 
 class Manifest
 {
-    public const string MANIFEST_FILENAME = 'manifest.json';
+    public const MANIFEST_FILENAME = 'manifest.json';
 
     /**
      * [manifest file dirname => [src file => target file]]