diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ebb9e1a3..5cb6ffd2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [5.4, 5.5, 5.6, 7.0, 7.1, 7.2, 7.3, 7.4, 8.0, 8.1]
+        php: ['5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
 
     name: PHP ${{ matrix.php }}