Add HTMLPurifier to extlib

HTMLPurifier defangs arbitrary submitted HTML. We're using it in the
OStatus plugin, but it may be valuable for other parts of the codebase
(I think OEmbed might benefit, for example).

extlib/HTMLPurifier/HTMLPurifier/ChildDef.php

@@ -0,0 +1,48 @@
+<?php
+
+/**
+ * Defines allowed child nodes and validates tokens against it.
+ */
+abstract class HTMLPurifier_ChildDef
+{
+    /**
+     * Type of child definition, usually right-most part of class name lowercase.
+     * Used occasionally in terms of context.
+     */
+    public $type;
+
+    /**
+     * Bool that indicates whether or not an empty array of children is okay
+     *
+     * This is necessary for redundant checking when changes affecting
+     * a child node may cause a parent node to now be disallowed.
+     */
+    public $allow_empty;
+
+    /**
+     * Lookup array of all elements that this definition could possibly allow
+     */
+    public $elements = array();
+
+    /**
+     * Get lookup of tag names that should not close this element automatically.
+     * All other elements will do so.
+     */
+    public function getAllowedElements($config) {
+        return $this->elements;
+    }
+
+    /**
+     * Validates nodes according to definition and returns modification.
+     *
+     * @param $tokens_of_children Array of HTMLPurifier_Token
+     * @param $config HTMLPurifier_Config object
+     * @param $context HTMLPurifier_Context object
+     * @return bool true to leave nodes as is
+     * @return bool false to remove parent node
+     * @return array of replacement child tokens
+     */
+    abstract public function validateChildren($tokens_of_children, $config, $context);
+}
+
+// vim: et sw=4 sts=4