Minor rewording

Author: wouterj

book/performance.rst

@@ -19,7 +19,7 @@ One of the best (and easiest) things that you should do to improve your performa
 is to use a "byte code cache". The idea of a byte code cache is to remove
 the need to constantly recompile the PHP source code. There are a number of
 `byte code caches`_ available, some of which are open source. If you are using 
-PHP 5.5 or more, the byte code cache is built-in into PHP runtime. For older versions, 
+PHP 5.5 or higher, `OPcache`_ is built-in into PHP runtime. For older versions,
 the most widely used byte code cache is probably `APC`_
 
 Using a byte code cache really has no downside, and Symfony has been architected
@@ -140,6 +140,7 @@ feature is disabled in the byte code cache (e.g. ``apc.stat=0`` in APC), there
 is no longer a reason to use a bootstrap file.
 
 .. _`byte code caches`: http://en.wikipedia.org/wiki/List_of_PHP_accelerators
+.. _`OPcache`: http://php.net/manual/en/book.opcache.php
 .. _`APC`: http://php.net/manual/en/book.apc.php
 .. _`autoload.php`: https://github.com/symfony/symfony-standard/blob/master/app/autoload.php
 .. _`bootstrap file`: https://github.com/sensio/SensioDistributionBundle/blob/master/Composer/ScriptHandler.php