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/ErrorStruct.php

@@ -0,0 +1,60 @@
+<?php
+
+/**
+ * Records errors for particular segments of an HTML document such as tokens,
+ * attributes or CSS properties. They can contain error structs (which apply
+ * to components of what they represent), but their main purpose is to hold
+ * errors applying to whatever struct is being used.
+ */
+class HTMLPurifier_ErrorStruct
+{
+
+    /**
+     * Possible values for $children first-key. Note that top-level structures
+     * are automatically token-level.
+     */
+    const TOKEN     = 0;
+    const ATTR      = 1;
+    const CSSPROP   = 2;
+
+    /**
+     * Type of this struct.
+     */
+    public $type;
+
+    /**
+     * Value of the struct we are recording errors for. There are various
+     * values for this:
+     *  - TOKEN: Instance of HTMLPurifier_Token
+     *  - ATTR: array('attr-name', 'value')
+     *  - CSSPROP: array('prop-name', 'value')
+     */
+    public $value;
+
+    /**
+     * Errors registered for this structure.
+     */
+    public $errors = array();
+
+    /**
+     * Child ErrorStructs that are from this structure. For example, a TOKEN
+     * ErrorStruct would contain ATTR ErrorStructs. This is a multi-dimensional
+     * array in structure: [TYPE]['identifier']
+     */
+    public $children = array();
+
+    public function getChild($type, $id) {
+        if (!isset($this->children[$type][$id])) {
+            $this->children[$type][$id] = new HTMLPurifier_ErrorStruct();
+            $this->children[$type][$id]->type = $type;
+        }
+        return $this->children[$type][$id];
+    }
+
+    public function addError($severity, $message) {
+        $this->errors[] = array($severity, $message);
+    }
+
+}
+
+// vim: et sw=4 sts=4