Handle PHP-bug with single char vars (#205)

We encounter an issue with single-char PHP vars in production (running on amphp/aerys). We observe regularly, that single-char-vars become undefined. After PHP restart everything is ok again. The bug definitely exists in 7.1.9 + 7.1.11
Fixing that bug is hard, a workaround is simple and does not harm.

Thanks!

Author: brstgt

CacheItem.php

@@ -240,9 +240,9 @@ private function tag($tags)
     private function initialize()
     {
         if ($this->callable !== null) {
-            // $f will be $adapter->fetchObjectFromCache();
-            $f                         = $this->callable;
-            $result                    = $f();
+            // $func will be $adapter->fetchObjectFromCache();
+            $func                      = $this->callable;
+            $result                    = $func();
             $this->hasValue            = $result[0];
             $this->value               = $result[1];
             $this->prevTags            = isset($result[2]) ? $result[2] : [];