hhvm RecursiveTreeIterator support

This code is from the PHP source itself. It's required to run
Respect\Validation on hhvm, which lacks the class.

By adding it to the library folder, it's loaded automatically
by our testing routines.

Further documentation should be written, but only after a stable
build for hhvm appears on Travis.

library/RecursiveTreeIterator.php

@@ -0,0 +1,135 @@
+<?php
+
+/** Required for hhvm compatibility, can be ignored for Zend PHP.
+  *
+  * Code from PHP itself: https://raw.githubusercontent.com/php/php-src/master/ext/spl/internal/recursivetreeiterator.inc
+  */
+
+/** @file recursivetreeiterator.inc
+ * @ingroup SPL
+ * @brief   class RecursiveTreeIterator
+ * @author  Marcus Boerger, Johannes Schlueter
+ * @date    2005 - 2009
+ *
+ * SPL - Standard PHP Library
+ */
+
+
+/** @ingroup SPL
+ * @brief   RecursiveIteratorIterator to generate ASCII graphic trees for the
+ *          entries in a RecursiveIterator
+ * @author  Marcus Boerger, Johannes Schlueter
+ * @version 1.1
+ * @since   PHP 5.3
+ */
+class RecursiveTreeIterator extends RecursiveIteratorIterator
+{
+    const BYPASS_CURRENT = 0x00000004;
+    const BYPASS_KEY     = 0x00000008;
+
+    private $rit_flags;
+
+    /**
+     * @param it         iterator to use as inner iterator
+     * @param rit_flags  flags passed to RecursiveIteratoIterator (parent)
+     * @param cit_flags  flags passed to RecursiveCachingIterator (for hasNext)
+     * @param mode       mode  passed to RecursiveIteratoIterator (parent)
+     */
+    function __construct(RecursiveIterator $it, $rit_flags = self::BYPASS_KEY, $cit_flags = CachingIterator::CATCH_GET_CHILD, $mode = self::SELF_FIRST)
+    {
+        parent::__construct(new RecursiveCachingIterator($it, $cit_flags), $mode, $rit_flags);
+        $this->rit_flags = $rit_flags;
+    }
+
+    private $prefix = array(0=>'', 1=>'| ', 2=>'  ', 3=>'|-', 4=>'\-', 5=>'');
+
+    /** Prefix used to start elements. */
+    const PREFIX_LEFT         = 0;
+    /** Prefix used if $level < depth and hasNext($level) == true. */
+    const PREFIX_MID_HAS_NEXT = 1;
+    /** Prefix used if $level < depth and hasNext($level) == false. */
+    const PREFIX_MID_LAST     = 2;
+    /** Prefix used if $level == depth and hasNext($level) == true. */
+    const PREFIX_END_HAS_NEXT = 3;
+    /** Prefix used if $level == depth and hasNext($level) == false. */
+    const PREFIX_END_LAST     = 4;
+    /** Prefix used right in front of the current element. */
+    const PREFIX_RIGHT        = 5;
+
+    /**
+     * Set prefix part as used in getPrefix() and stored in $prefix.
+     * @param $part   any PREFIX_* const.
+     * @param $value  new prefix string for specified part.
+     * @throws OutOfRangeException if 0 > $part or $part > 5.
+     */
+    function setPrefixPart($part, $value)
+    {
+        if (0 > $part || $part > 5) {
+            throw new OutOfRangeException();
+        }
+        $this->prefix[$part] = (string)$value;
+    }
+
+    /** @return string to place in front of current element
+     */
+    function getPrefix()
+    {
+        $tree = '';
+        for ($level = 0; $level < $this->getDepth(); $level++)
+        {
+            $tree .= $this->getSubIterator($level)->hasNext() ? $this->prefix[1] : $this->prefix[2];
+        }
+        $tree .= $this->getSubIterator($level)->hasNext() ? $this->prefix[3] : $this->prefix[4];
+
+        return $this->prefix[0] . $tree . $this->prefix[5];
+    }
+
+    /** @return string presentation build for current element
+     */
+    function getEntry()
+    {
+        return @(string)parent::current();
+    }
+
+    /** @return string to place after the current element
+     */
+    function getPostfix()
+    {
+        return '';
+    }
+
+    /** @return the current element prefixed and postfixed
+     */
+    function current()
+    {
+        if ($this->rit_flags & self::BYPASS_CURRENT)
+        {
+            return parent::current();
+        }
+        else
+        {
+            return $this->getPrefix() . $this->getEntry() .  $this->getPostfix();
+        }
+    }
+
+    /** @return the current key prefixed and postfixed
+     */
+    function key()
+    {
+        if ($this->rit_flags & self::BYPASS_KEY)
+        {
+            return parent::key();
+        }
+        else
+        {
+            return $this->getPrefix() . parent::key() .  $this->getPostfix();
+        }
+    }
+
+    /** Aggregates the inner iterator
+     */
+    function __call($func, $params)
+    {
+        return call_user_func_array(array($this->getSubIterator(), $func), $params);
+    }
+}