diff --git a/class.smtp.php b/class.smtp.php
index 2c5b8d300..2e6003131 100644
--- a/class.smtp.php
+++ b/class.smtp.php
@@ -188,7 +188,8 @@ protected function edebug($str, $level = 0)
         if ($level > $this->do_debug) {
             return;
         }
-        if (is_callable($this->Debugoutput)) {
+        //Avoid clash with built-in function names
+        if (!in_array($this->Debugoutput, array('error_log', 'html', 'echo')) and is_callable($this->Debugoutput)) {
             call_user_func($this->Debugoutput, $str, $this->do_debug);
             return;
         }