make OPcache optional, resolves #1678

doc/Installation.md

@@ -30,7 +30,7 @@ for more information.
 ### Optional Requirements
 
 - PHP with GD extension (when using identicon or vizhash icons, jdenticon works
-  without it)
+  without it) and OPcache (for better performance)
 - a database supported by [PHP PDO](https://php.net/manual/book.pdo.php) and the
   PHP PDO extension (when using database storage)
 - a Ceph cluster with Rados gateway or AWS S3 storage (when using S3 storage)

lib/Data/Filesystem.php

@@ -269,7 +269,9 @@ class Filesystem extends AbstractData
     {
         switch ($namespace) {
             case 'purge_limiter':
-                opcache_invalidate($this->_path . DIRECTORY_SEPARATOR . 'purge_limiter.php');
+                if (function_exists('opcache_invalidate')) {
+                    opcache_invalidate($this->_path . DIRECTORY_SEPARATOR . 'purge_limiter.php');
+                }
                 return $this->_storeString(
                     $this->_path . DIRECTORY_SEPARATOR . 'purge_limiter.php',
                     '<?php' . PHP_EOL . '$GLOBALS[\'purge_limiter\'] = ' . var_export($value, true) . ';'
@@ -281,7 +283,9 @@ class Filesystem extends AbstractData
                 );
             case 'traffic_limiter':
                 $this->_last_cache[$key] = $value;
-                opcache_invalidate($this->_path . DIRECTORY_SEPARATOR . 'traffic_limiter.php');
+                if (function_exists('opcache_invalidate')) {
+                    opcache_invalidate($this->_path . DIRECTORY_SEPARATOR . 'traffic_limiter.php');
+                }
                 return $this->_storeString(
                     $this->_path . DIRECTORY_SEPARATOR . 'traffic_limiter.php',
                     '<?php' . PHP_EOL . '$GLOBALS[\'traffic_limiter\'] = ' . var_export($this->_last_cache, true) . ';'