Implement attribute transforms for required attributes. I can now confidently say that output will always be valid.

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@256 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/AttrTransform/ImgRequired.php

@@ -0,0 +1,47 @@
+<?php
+
+require_once 'HTMLPurifier/AttrTransform.php';
+
+// must be called POST validation
+
+HTMLPurifier_ConfigDef::define(
+    'Attr', 'DefaultInvalidImage', '',
+    'This is the default image an img tag will be pointed to if it does '.
+    'not have a valid src attribute.  In future versions, we may allow the '.
+    'image tag to be removed completely, but due to design issues, this is '.
+    'not possible right now.'
+);
+
+HTMLPurifier_ConfigDef::define(
+    'Attr', 'DefaultInvalidImageAlt', 'Invalid image',
+    'This is the content of the alt tag of an invalid image if the user '.
+    'had not previously specified an alt attribute.  It has no effect when the '.
+    'image is valid but there was no alt attribute present.'
+);
+
+class HTMLPurifier_AttrTransform_ImgRequired extends HTMLPurifier_AttrTransform
+{
+    
+    function transform($attributes, $config) {
+        
+        $src = true;
+        if (!isset($attributes['src'])) {
+            $attributes['src'] = $config->get('Attr', 'DefaultInvalidImage');
+            $src = false;
+        }
+        
+        if (!isset($attributes['alt'])) {
+            if ($src) {
+                $attributes['alt'] = basename($attributes['src']);
+            } else {
+                $attributes['alt'] = $config->get('Attr', 'DefaultInvalidImageAlt');
+            }
+        }
+        
+        return $attributes;
+        
+    }
+    
+}
+
+?>