PECL Redis 2.2.8 was the last version to support PHP 5.x

plugins/templates/before-script-redis.yml

@@ -16,5 +16,10 @@ before_script:
   - composer global require yoast/phpunit-polyfills
 
   # PECL Redis module
-  - pecl install redis
+  - |
+    if [[ $(php -v) =~ "PHP 5.6" ]]; then
+      pecl install redis-2.2.8
+    elif
+      pecl install redis
+    fi
   - echo "extension=redis.so" > /usr/local/etc/php/conf.d/redis.ini