diff --git a/src/voku/helper/UTF8.php b/src/voku/helper/UTF8.php
index 78119a49..96c7cb9d 100644
--- a/src/voku/helper/UTF8.php
+++ b/src/voku/helper/UTF8.php
@@ -1280,6 +1280,16 @@ public static function encode($encoding, $str, $force = true)
         return self::to_iso8859($str);
       }
 
+      if (
+          $encoding !== 'UTF-8'
+          &&
+          $encoding !== 'WINDOWS-1252'
+          &&
+          self::$support['mbstring'] === false
+      ) {
+        trigger_error('UTF8::encode() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+      }
+
       $strEncoded = \mb_convert_encoding(
           $str,
           $encoding,
@@ -4727,10 +4737,14 @@ public static function stristr($haystack, $needle, $before_needle = false, $enco
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
+      trigger_error('UTF8::stristr() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
       return \mb_stristr($haystack, $needle, $before_needle, $encoding);
     }
 
@@ -4792,10 +4806,14 @@ public static function strlen($str, $encoding = 'UTF-8', $cleanUtf8 = false)
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
+      trigger_error('UTF8::strlen() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
       return \mb_strlen($str, $encoding);
     }
 
@@ -4813,6 +4831,7 @@ public static function strlen($str, $encoding = 'UTF-8', $cleanUtf8 = false)
       }
     }
 
+    // fallback via vanilla php
     preg_match_all('/./us', $str, $parts);
     $returnTmp = count($parts[0]);
     if ($returnTmp !== 0) {
@@ -4980,14 +4999,22 @@ public static function strpos($haystack, $needle, $offset = 0, $encoding = 'UTF-
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
+      trigger_error('UTF8::strpos() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
       return \mb_strpos($haystack, $needle, $offset, $encoding);
     }
 
-    if (self::$support['iconv'] === true) {
+    if (
+        $offset >= 0 // iconv_strpos() can't handle negative offset
+        &&
+        self::$support['iconv'] === true
+    ) {
       // ignore invalid negative offset to keep compatibility
       // with php < 5.5.35, < 5.6.21, < 7.0.6
       return \iconv_strpos($haystack, $needle, $offset > 0 ? $offset : 0, $encoding);
@@ -5013,7 +5040,13 @@ public static function strpos($haystack, $needle, $offset = 0, $encoding = 'UTF-
       return false;
     }
 
-    return $offset + self::strlen(substr($haystack, 0, $pos));
+    $returnTmp = $offset + self::strlen(substr($haystack, 0, $pos));
+    if ($returnTmp !== false) {
+      return $returnTmp;
+    }
+
+    // fallback to "mb_"-function via polyfill
+    return \mb_strpos($haystack, $needle, $offset);
   }
 
   /**
@@ -5052,6 +5085,7 @@ public static function strrchr($haystack, $needle, $before_needle = false, $enco
       $haystack = self::clean($haystack);
     }
 
+    // fallback to "mb_"-function via polyfill
     return \mb_strrchr($haystack, $needle, $before_needle, $encoding);
   }
 
@@ -5167,10 +5201,14 @@ public static function strripos($haystack, $needle, $offset = 0, $encoding = 'UT
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
+      trigger_error('UTF8::strripos() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
       return \mb_strripos($haystack, $needle, $offset, $encoding);
     }
 
@@ -5181,6 +5219,8 @@ public static function strripos($haystack, $needle, $offset = 0, $encoding = 'UT
       }
     }
 
+    // fallback via vanilla php
+
     return self::strrpos(self::strtolower($haystack, $encoding), self::strtolower($needle, $encoding), $offset, $encoding, $cleanUtf8);
   }
 
@@ -5242,11 +5282,18 @@ public static function strrpos($haystack, $needle, $offset = null, $encoding = '
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
-      return \mb_strrpos($haystack, $needle, $offset, $encoding);
+      trigger_error('UTF8::strrpos() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
+      $returnTmp = \mb_strrpos($haystack, $needle, $offset, $encoding);
+      if ($returnTmp !== false) {
+        return $returnTmp;
+      }
     }
 
     if (self::$support['intl'] === true) {
@@ -5341,15 +5388,25 @@ public static function strstr($haystack, $needle, $before_needle = false, $encod
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
-      return \mb_strstr($haystack, $needle, $before_needle, $encoding);
+      trigger_error('UTF8::strstr() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
+      $returnTmp = \mb_strstr($haystack, $needle, $before_needle, $encoding);
+      if ($returnTmp !== false) {
+        return $returnTmp;
+      }
     }
 
     if (self::$support['intl'] === true) {
-      return \grapheme_strstr($haystack, $needle, $before_needle);
+      $returnTmp = \grapheme_strstr($haystack, $needle, $before_needle);
+      if ($returnTmp !== false) {
+        return $returnTmp;
+      }
     }
 
     preg_match('/^(.*?)' . preg_quote($needle, '/') . '/us', $haystack, $match);
@@ -5551,6 +5608,7 @@ public static function strwidth($str, $encoding = 'UTF-8', $cleanUtf8 = false)
       $str = self::clean($str);
     }
 
+    // fallback to "mb_"-function via polyfill
     return \mb_strwidth($str, $encoding);
   }
 
@@ -5613,15 +5671,23 @@ public static function substr($str, $start = 0, $length = null, $encoding = 'UTF
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
+      trigger_error('UTF8::substr() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
       return \mb_substr($str, $start, $length, $encoding);
     }
 
-    if (self::$support['iconv'] === true) {
-      return \iconv_substr($str, $start, $length, $encoding);
+    if (
+        $length >= 0 // "iconv_substr()" can't handle negative length
+        &&
+        self::$support['iconv'] === true
+    ) {
+      return \iconv_substr($str, $start, $length);
     }
 
     if (self::$support['intl'] === true) {
@@ -5714,10 +5780,14 @@ public static function substr_count($haystack, $needle, $offset = 0, $length = n
     }
 
     if (
-        $encoding !== 'UTF-8' // INFO: use "mb_"-function (with polyfill) also if we need another encoding
-        ||
-        self::$support['mbstring'] === true
+        $encoding !== 'UTF-8'
+        &&
+        self::$support['mbstring'] === false
     ) {
+      trigger_error('UTF8::substr_count() without mbstring cannot handle "' . $encoding . '" encoding', E_USER_WARNING);
+    }
+
+    if (self::$support['mbstring'] === true) {
       return \mb_substr_count($haystack, $needle, $encoding);
     }
 
@@ -5736,6 +5806,7 @@ public static function substr_count($haystack, $needle, $offset = 0, $length = n
    */
   public static function substr_ileft($haystack, $needle)
   {
+    // init
     $haystack = (string)$haystack;
     $needle = (string)$needle;
 
@@ -5764,6 +5835,7 @@ public static function substr_ileft($haystack, $needle)
    */
   public static function substr_iright($haystack, $needle)
   {
+    // init
     $haystack = (string)$haystack;
     $needle = (string)$needle;
 
@@ -5792,6 +5864,7 @@ public static function substr_iright($haystack, $needle)
    */
   public static function substr_left($haystack, $needle)
   {
+    // init
     $haystack = (string)$haystack;
     $needle = (string)$needle;
 
@@ -5815,12 +5888,24 @@ public static function substr_left($haystack, $needle)
    *
    * source: https://gist.github.com/stemar/8287074
    *
-   * @param string|string[] $str         <p>The input string or an array of stings.</p>
-   * @param string|string[] $replacement <p>The replacement string or an array of stings.</p>
-   * @param int|int[]       $start
-   * @param int|int[]|void  $length      [optional]
-   *
-   * @return string|string[]
+   * @param string|string[] $str              <p>The input string or an array of stings.</p>
+   * @param string|string[] $replacement      <p>The replacement string or an array of stings.</p>
+   * @param int|int[]       $start            <p>
+   *                                          If start is positive, the replacing will begin at the start'th offset
+   *                                          into string.
+   *                                          <br /><br />
+   *                                          If start is negative, the replacing will begin at the start'th character
+   *                                          from the end of string.
+   *                                          </p>
+   * @param int|int[]|void  $length           [optional] <p>If given and is positive, it represents the length of the
+   *                                          portion of string which is to be replaced. If it is negative, it
+   *                                          represents the number of characters from the end of string at which to
+   *                                          stop replacing. If it is not given, then it will default to strlen(
+   *                                          string ); i.e. end the replacing at the end of string. Of course, if
+   *                                          length is zero then this function will have the effect of inserting
+   *                                          replacement into string at the given start offset.</p>
+   *
+   * @return string|string[] <p>The result string is returned. If string is an array then array is returned.</p>
    */
   public static function substr_replace($str, $replacement, $start, $length = null)
   {
@@ -5864,7 +5949,9 @@ public static function substr_replace($str, $replacement, $start, $length = null
 
       // Recursive call
       return array_map(array(__CLASS__, 'substr_replace'), $str, $replacement, $start, $length);
+
     } else {
+
       if (is_array($replacement)) {
         if (count($replacement) > 0) {
           $replacement = $replacement[0];
@@ -5874,11 +5961,19 @@ public static function substr_replace($str, $replacement, $start, $length = null
       }
     }
 
-    preg_match_all('/./us', (string)$str, $smatches);
-    preg_match_all('/./us', (string)$replacement, $rmatches);
+    // init
+    $str = (string)$str;
+    $replacement = (string)$replacement;
+
+    if (!isset($str[0])) {
+      return $replacement;
+    }
+
+    preg_match_all('/./us', $str, $smatches);
+    preg_match_all('/./us', $replacement, $rmatches);
 
     if ($length === null) {
-      $length = (int)\mb_strlen($str);
+      $length = (int)self::strlen($str);
     }
 
     array_splice($smatches[0], $start, $length, $rmatches[0]);
@@ -6308,13 +6403,7 @@ function ($match) {
     );
 
     // decode UTF-8 codepoints
-    $buf = preg_replace_callback(
-        '/&#\d{2,6};/',
-        function ($match) {
-          return \mb_convert_encoding($match[0], 'UTF-8', 'HTML-ENTITIES');
-        },
-        $buf
-    );
+    $buf = self::html_entity_decode($buf, ENT_QUOTES);
 
     return $buf;
   }
diff --git a/tests/Utf8GlobalTest.php b/tests/Utf8GlobalTest.php
index 71b1100a..7cf3b454 100644
--- a/tests/Utf8GlobalTest.php
+++ b/tests/Utf8GlobalTest.php
@@ -515,8 +515,10 @@ public function testEncode()
         'Berbée'         => 'Berb?e',
     );
 
-    foreach ($tests as $before => $after) {
-      self::assertSame($after, UTF8::encode('CP367', $before), 'tested: ' . $before); // CP367
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      foreach ($tests as $before => $after) {
+        self::assertSame($after, UTF8::encode('CP367', $before), 'tested: ' . $before); // CP367
+      }
     }
 
     $tests = array(
@@ -1112,7 +1114,6 @@ public function testHtmlEntityDecodeWithEntQuotes()
         'who&amp;#039;s online'          => 'who\'s online',
         'who&#039;s online-'             => 'who\'s online-',
         'Who&#039;s Online'              => 'Who\'s Online',
-        'Who&amp;#039;s Online &#20013;' => 'Who\'s Online ?',
         'Who&amp;amp;#039;s Online'      => 'Who\'s Online',
         "Who\'s Online&#x0003A;"         => 'Who\\\'s Online:',
     );
@@ -1120,6 +1121,12 @@ public function testHtmlEntityDecodeWithEntQuotes()
     foreach ($testArray as $before => $after) {
       self::assertSame($after, UTF8::html_entity_decode($before, ENT_QUOTES, 'ISO'), 'error by ' . $before); // 'ISO-8859-1'
     }
+
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame('Who\'s Online ?', UTF8::html_entity_decode('Who&amp;#039;s Online &#20013;', ENT_QUOTES, 'ISO'));
+    } else {
+      self::assertSame('Who\'s Online ', UTF8::html_entity_decode('Who&amp;#039;s Online &#20013;', ENT_QUOTES, 'ISO'));
+    }
   }
 
   public function testHtmlEntityDecodeWithHtml5()
@@ -2492,10 +2499,13 @@ public function testStrlen()
 
     self::assertSame(74, strlen($string));
     self::assertSame(74, UTF8::strlen($string, '8bit'));
-    self::assertSame(71, UTF8::strlen($string));
-    self::assertSame(71, UTF8::strlen($string, 'UTF-8', false));
     self::assertSame(67, UTF8::strlen($string, 'UTF-8', true));
 
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame(71, UTF8::strlen($string));
+      self::assertSame(71, UTF8::strlen($string, 'UTF-8', false));
+    }
+
     $string_test1 = strip_tags($string);
     $string_test2 = UTF8::strip_tags($string);
 
@@ -2694,31 +2704,33 @@ public function testStrpos()
 
       // --- invalid UTF-8
 
-      self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白'));
+      if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+        self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白'));
 
-      if (Bootup::is_php('7.1') === false) {
-        self::assertSame(false, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -8));
-      } else {
-        self::assertSame(20, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -8));
-      }
+        if (Bootup::is_php('7.1') === false) {
+          self::assertSame(false, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -8));
+        } else {
+          self::assertSame(20, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -8));
+        }
 
-      self::assertSame(false, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -4));
-      self::assertSame(false, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -1));
-      self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0));
-      self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 4));
-      self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 8));
-      self::assertSame(14, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'UTF-8', true));
-      self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'UTF-8', false));
-      self::assertSame(26, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'ISO', true));
-      self::assertSame(27, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'ISO', false));
+        self::assertSame(false, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -4));
+        self::assertSame(false, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', -1));
+        self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0));
+        self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 4));
+        self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 8));
+        self::assertSame(14, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'UTF-8', true));
+        self::assertSame(15, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'UTF-8', false));
+        self::assertSame(26, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'ISO', true));
+        self::assertSame(27, UTF8::strpos('ABC-ÖÄÜ-💩-' . "\xc3\x28" . '中文空白-中文空白' . "\xf0\x28\x8c\x28" . 'abc', '白', 0, 'ISO', false));
 
-      // ISO
+        // ISO
 
-      self::assertSame(17, strpos('der Straße nach Paris', 'Paris', 0)); // not correct
-      self::assertSame(17, UTF8::strpos('der Straße nach Paris', 'Paris', 0, 'ISO')); // not correct
+        self::assertSame(17, strpos('der Straße nach Paris', 'Paris', 0)); // not correct
+        self::assertSame(17, UTF8::strpos('der Straße nach Paris', 'Paris', 0, 'ISO')); // not correct
 
-      self::assertSame(3, strpos('한국어', '국', 0)); // not correct
-      self::assertSame(3, UTF8::strpos('한국어', '국', 0, 'ISO')); // not correct
+        self::assertSame(3, strpos('한국어', '국', 0)); // not correct
+        self::assertSame(3, UTF8::strpos('한국어', '국', 0, 'ISO')); // not correct
+      }
     }
   }
 
@@ -2749,11 +2761,13 @@ public function testStrrchr()
 
     // --- ISO
 
-    self::assertSame('κόσμε-äöü', UTF8::strrchr('κόσμεκόσμε-äöü', 'κόσμε', false, 'ISO'));
-    self::assertSame(false, UTF8::strrchr('Aκόσμεκόσμε-äöü', 'aκόσμε', false, 'ISO'));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame('κόσμε-äöü', UTF8::strrchr('κόσμεκόσμε-äöü', 'κόσμε', false, 'ISO'));
+      self::assertSame(false, UTF8::strrchr('Aκόσμεκόσμε-äöü', 'aκόσμε', false, 'ISO'));
 
-    self::assertSame('κόσμε', UTF8::strrchr('κόσμεκόσμε-äöü', 'κόσμε', true, 'ISO'));
-    self::assertSame(false, UTF8::strrchr('Aκόσμεκόσμε-äöü', 'aκόσμε', true, 'ISO'));
+      self::assertSame('κόσμε', UTF8::strrchr('κόσμεκόσμε-äöü', 'κόσμε', true, 'ISO'));
+      self::assertSame(false, UTF8::strrchr('Aκόσμεκόσμε-äöü', 'aκόσμε', true, 'ISO'));
+    }
   }
 
   public function testStrrev()
@@ -2799,12 +2813,13 @@ public function testStrrichr()
 
     // --- ISO
 
-    self::assertSame('Aκόσμεκόσμε-äöü', UTF8::strrichr('Aκόσμεκόσμε-äöü', 'aκόσμε', false, 'ISO'));
-    self::assertSame('ü-abc', UTF8::strrichr('äöü-abc', 'ü', false, 'ISO'));
-
-    self::assertSame('', UTF8::strrichr('Aκόσμεκόσμε-äöü', 'aκόσμε', true, 'ISO'));
-    self::assertSame('äö', UTF8::strrichr('äöü-abc', 'ü', true, 'ISO'));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame('Aκόσμεκόσμε-äöü', UTF8::strrichr('Aκόσμεκόσμε-äöü', 'aκόσμε', false, 'ISO'));
+      self::assertSame('ü-abc', UTF8::strrichr('äöü-abc', 'ü', false, 'ISO'));
 
+      self::assertSame('', UTF8::strrichr('Aκόσμεκόσμε-äöü', 'aκόσμε', true, 'ISO'));
+      self::assertSame('äö', UTF8::strrichr('äöü-abc', 'ü', true, 'ISO'));
+    }
   }
 
   public function testStrrpos()
@@ -2816,11 +2831,14 @@ public function testStrrpos()
       self::assertSame(1, UTF8::strrpos('한국어', '국', 0, '8bit', false));
       self::assertSame(1, UTF8::strrpos('한국어', '국', 0, 'ISO', false));
       self::assertSame(1, UTF8::strrpos('한국어', '국', 0, '', true));
-
     } else {
-      self::assertSame(3, UTF8::strrpos('한국어', '국', 0, '8bit', false));
-      self::assertSame(3, UTF8::strrpos('한국어', '국', 0, 'ISO', false));
-      self::assertSame(false, UTF8::strrpos('한국어', '국', 0, '', true));
+
+      if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+        self::assertSame(3, UTF8::strrpos('한국어', '국', 0, '8bit', false));
+        self::assertSame(3, UTF8::strrpos('한국어', '국', 0, 'ISO', false));
+      }
+
+      self::assertSame(1, UTF8::strrpos('한국어', '국', 0, '', true));
     }
 
     self::assertSame(1, UTF8::strrpos('한국어', '국', 0, 'UTF-8', false));
@@ -2855,10 +2873,11 @@ public function testStrtocasefold()
 
     // invalid utf-8
 
-    if (Bootup::is_php('5.4')) {
-      // invalid UTF-8 + PHP 5.3 = 20 => error
-      self::assertSame('iñtërnâtiôn?àlizætiøn', UTF8::strtocasefold("Iñtërnâtiôn\xE9àlizætiøn"));
-      self::assertSame('iñtërnâtiôn?àlizætiøn', UTF8::strtocasefold("Iñtërnâtiôn\xE9àlizætiøn", true));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      if (Bootup::is_php('5.4')) { // invalid UTF-8 + PHP 5.3 = 20 => error
+        self::assertSame('iñtërnâtiôn?àlizætiøn', UTF8::strtocasefold("Iñtërnâtiôn\xE9àlizætiøn"));
+        self::assertSame('iñtërnâtiôn?àlizætiøn', UTF8::strtocasefold("Iñtërnâtiôn\xE9àlizætiøn", true));
+      }
     }
 
     self::assertSame('iñtërnâtiônàlizætiøn', UTF8::strtocasefold("Iñtërnâtiôn\xE9àlizætiøn", true, true));
@@ -2970,20 +2989,26 @@ public function testStrwidth()
 
     // test + Invalid Chars
 
-    if (Bootup::is_php('5.4')) {
-      // invalid UTF-8 + PHP 5.3 = 20 => error
-      self::assertSame(21, UTF8::strwidth("Iñtërnâtiôn\xE9àlizætiøn", 'UTF8', false));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      if (Bootup::is_php('5.4')) { // invalid UTF-8 + PHP 5.3 = 20 => error
+        self::assertSame(21, UTF8::strwidth("Iñtërnâtiôn\xE9àlizætiøn", 'UTF8', false));
+      }
     }
 
     self::assertSame(20, UTF8::strwidth("Iñtërnâtiôn\xE9àlizætiøn", 'UTF8', true));
 
-    self::assertSame(20, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'UTF8', false));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame(20, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'UTF8', false));
+    }
+
     self::assertSame(20, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'UTF8', true));
 
     // ISO
 
-    self::assertSame(28, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'ISO', false));
-    self::assertSame(27, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'ISO', true));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame(28, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'ISO', false));
+      self::assertSame(27, UTF8::strlen("Iñtërnâtiôn\xE9àlizætiøn", 'ISO', true));
+    }
   }
 
   public function testSubstr()
@@ -3103,8 +3128,10 @@ public function testSubstrCount()
 
     // ISO
 
-    self::assertSame(0, UTF8::substr_count('中文空白', '文空', 1, 2, 'ISO'));
-    self::assertSame(1, UTF8::substr_count('abcde', 'bc', 1, 2, 'ISO'));
+    if (UTF8::mbstring_loaded() === true) { // only with "mbstring"
+      self::assertSame(0, UTF8::substr_count('中文空白', '文空', 1, 2, 'ISO'));
+      self::assertSame(1, UTF8::substr_count('abcde', 'bc', 1, 2, 'ISO'));
+    }
   }
 
   public function testSubstrILeft()
diff --git a/tests/Utf8StristrTest.php b/tests/Utf8StristrTest.php
index c3de39c4..efda4896 100644
--- a/tests/Utf8StristrTest.php
+++ b/tests/Utf8StristrTest.php
@@ -82,8 +82,10 @@ public function test_encoding()
     // UTF-8
     self::assertSame("n\nâtiônàlizætiøn", u::stristr($str, $search, 0, 'UTF-8', false));
 
-    // UTF-7
-    self::assertSame("n\n??ti??n??liz??ti??n", u::stristr($str, $search, 0, 'UTF-7', false));
+    if (u::mbstring_loaded() === true) { // only with "mbstring"
+      // UTF-7
+      self::assertSame("n\n??ti??n??liz??ti??n", u::stristr($str, $search, 0, 'UTF-7', false));
+    }
   }
 
   public function test_clean_utf8()
diff --git a/tests/Utf8StrlenTest.php b/tests/Utf8StrlenTest.php
index 6c19c168..4ab9df43 100644
--- a/tests/Utf8StrlenTest.php
+++ b/tests/Utf8StrlenTest.php
@@ -15,8 +15,10 @@ public function test_utf8()
 
   public function test_utf8_invalid()
   {
-    $str = "Iñtërnâtiôn\xE9àlizætiøn";
-    self::assertSame(20, u::strlen($str));
+    if (u::mbstring_loaded() === true) { // only with "mbstring"
+      $str = "Iñtërnâtiôn\xE9àlizætiøn";
+      self::assertSame(20, u::strlen($str));
+    }
   }
 
   public function test_ascii()