diff --git a/lib/tests/weblib_test.php b/lib/tests/weblib_test.php
index 74362a4a64e..8ba4a4d5255 100644
--- a/lib/tests/weblib_test.php
+++ b/lib/tests/weblib_test.php
@@ -106,6 +106,22 @@ class web_testcase extends advanced_testcase {
             format_text_email('習習',FORMAT_HTML));
     }
 
+    function test_obfuscate_email() {
+        $email = 'some.user@example.com';
+        $obfuscated = obfuscate_email($email);
+        $this->assertNotSame($email, $obfuscated);
+        $back = textlib::entities_to_utf8(urldecode($email), true);
+        $this->assertSame($email, $back);
+    }
+
+    function test_obfuscate_text() {
+        $text = 'Žluťoučký koníček 32131';
+        $obfuscated = obfuscate_text($text);
+        $this->assertNotSame($text, $obfuscated);
+        $back = textlib::entities_to_utf8($obfuscated, true);
+        $this->assertSame($text, $back);
+    }
+
     function test_highlight() {
         $this->assertEquals(highlight('good', 'This is good'), 'This is <span class="highlight">good</span>');
         $this->assertEquals(highlight('SpaN', 'span'), '<span class="highlight">span</span>');
diff --git a/lib/weblib.php b/lib/weblib.php
index 933356710f0..cd43addb126 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -2594,19 +2594,20 @@ function redirect($url, $message='', $delay=-1) {
 function obfuscate_text($plaintext) {
 
     $i=0;
-    $length = strlen($plaintext);
+    $length = textlib::strlen($plaintext);
     $obfuscated='';
     $prev_obfuscated = false;
     while ($i < $length) {
-        $c = ord($plaintext{$i});
-        $numerical = ($c >= ord('0')) && ($c <= ord('9'));
+        $char = textlib::substr($plaintext, $i, 1);
+        $ord = textlib::utf8ord($char);
+        $numerical = ($ord >= ord('0')) && ($ord <= ord('9'));
         if ($prev_obfuscated and $numerical ) {
-            $obfuscated.='&#'.ord($plaintext{$i}).';';
+            $obfuscated.='&#'.$ord.';';
         } else if (rand(0,2)) {
-            $obfuscated.='&#'.ord($plaintext{$i}).';';
+            $obfuscated.='&#'.$ord.';';
             $prev_obfuscated = true;
         } else {
-            $obfuscated.=$plaintext{$i};
+            $obfuscated.=$char;
             $prev_obfuscated = false;
         }
       $i++;