php opening tags for syntax highlight

proposed/PSR-Cache.md

@@ -55,6 +55,7 @@ The main focus of the Cache\Pool object is to accept a key from the Calling Libr
 Items can be retrieved from the Cache\Pool individually using the getCache function, or in groups by retrieving an Iterator object from the getCacheIterator function. 
 
 ```php
+<?php
 namespace PSR\Cache;
 
 /**
@@ -103,6 +104,7 @@ interface Pool
 The Cache\Item object encapsulates the storage and retrieval of cache items. Each Cache\Item is generated by a Cache\Pool, which is responsible for any required setup as well as associating the object with a unique Key (how this is accomplished is the responsibility of the Implementing Library). Cache\Item objects can store and retrieve any type of PHP value defined in the Data section of this document.
 
 ```php
+<?php
 namespace PSR\Cache;
 
 /**
@@ -206,6 +208,7 @@ Stacks are a special kind of grouping system that allow cache items to be nested
 #### StackablePool
 
 ```php
+<?php
 namespace PSR\Cache\Extensions;
 
 /**
@@ -226,6 +229,7 @@ interface StackablePool extends \PSR\Cache\Pool
 #### StackableItem
 
 ```php
+<?php
 namespace PSR\Cache\Extensions;
 
 /**
@@ -251,6 +255,7 @@ Tagging interfaces are provided for completeness, but developers should note the
 #### Cache\Extensions\TaggablePool
 
 ```php
+<?php
 namespace PSR\Cache\Extensions;
 
 /**
@@ -276,6 +281,7 @@ interface TaggablePool extends \PSR\Cache\Pool
 #### Cache\Extensions\TaggableItem
 
 ```php
+<?php
 namespace PSR\Cache\Extensions;
 
 /**