Update phpunit version

php/composer.json

diff --git a/php/composer.json b/php/composer.json
index f712f0e..756704f 100644
--- a/php/composer.json
+++ b/php/composer.json
@@ -9,7 +9,7 @@
     "php": ">=7.0.0"
   },
   "require-dev": {
-    "phpunit/phpunit": ">=5.0.0"
+    "phpunit/phpunit": ">=5.0.0 <8.5.27"
   },
   "autoload": {
     "psr-4": {