Issue #315 Fix htmlspecialchars() warning for null values

Author: mikehaertl

src/XfdfFile.php

@@ -218,11 +218,14 @@ protected function writeFields($fp, $fields)
     }
 
     /**
-     * @param string $value the value to encode
-     * @return string the value correctly encoded for use in a XML document
+     * @param string|null $value the value to encode
+     * @return string|null the value correctly encoded for use in a XML document
      */
     protected function xmlEncode($value)
     {
+        if ($value === null) {
+            return null;
+        }
         return defined('ENT_XML1') ?
             htmlspecialchars($value, ENT_XML1, 'UTF-8') :
             htmlspecialchars($value);