Fatal Error when calling static method

So, here's my situation :

• I'm using CodeIgniter
• I've set up a helper, ('string_helper' under 'DK' folder)

• I've set up the dkString class in dk/string_helper.php

  static function strInArray($str,$arr)
  {
        foreach ($arr as $item) 
        {
              if (self::inString($str,$item))
                    return true;
        }
  
        return false;
  }
  

• In my controller:
  • I'm loading the helper ($this->load->helper('dk/string');)
  • Calling the method (dkString::strInArray($str,$arr);)

Note :

• I've loaded class's static methods residing in a custom helper, like 100 times. So there's nothing wrong with it.
• It doesn't matter whether the function is declared as static or not. Normally it works, no matter what.

However, I keep getting the following error :

Fatal error: Call to undefined method dkString::strInArray() in /the/path/to/the/caller/file.php on Line XX

Any ideas what might be wrong?

<?php

/*
 * DK4PHP Library
 *
 * Copyright (c) 2010-2012 by Dr.Kameleon
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published
 * by the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

if (!class_exists('dkString'))
{
    class dkString
    {
    /**
     * Return number of character in string
     * 
     * @param type $str
     * @return type 
     */

    function characterCount($str)
    {
        return strlen($str); 
    }

    /**
     * Check if str begins with...
     * @param type $str
     * @param type $sub
     * @return type 
     */

    function beginsWith($str, $sub)
    {
        return (substr($str,0,strlen($sub)) == $sub);
    }

    /**
     * Check if str ends with...
     * 
     * @param type $str
     * @param type $sub
     * @return type 
     */

    function endsWith($str, $sub)
    {
        return (substr($str,strlen($str)-strlen($sub)) == $sub);
    }

    /**
     * Check if str contains substring
     * 
     * @param type $sub
     * @param type $str
     * @param type $casesensitive
     * @return type 
     */

    function inString($sub, $str, $casesensitive = false)
    {
        if ($casesensitive)
        return (strstr($str, $sub) == false) ? false : true;
        else
        return (stristr($str, $sub) == false) ? false : true;
    }

    /**
     * Count number of occurences of substring in string
     * 
     * @param type $sub
     * @param type $str
     * @return type 
     */

    function inStringCount($sub, $str) 
    {
        return substr_count($str, $sub);
    }

    /**
     * Convert string to number
     * 
     * @param type $str
     * @param type $check
     * @param type $magic
     * @return type 
     */

    function strtonum($str, $check, $magic)
    {
        $int32Unit = 4294967296;

        $length = strlen($str);
        for ($i = 0; $i < $length; $i++)
        {
        $check *= $magic;
        if ($check >= $int32Unit)
        {
            $check = ($check - $int32Unit * (int) ($check / $int32Unit));

            $check = ($check < -2147483648) ? ($check + $int32Unit) : $check;
        }
        $check += ord($str{$i});
        }
        return $check;
    }

    /**
     * Get index of str in array (check if is contained)
     * 
     * @param type $str
     * @param type $arr 
     */

    function indexInArray($str,$arr)
    {
        foreach ($arr as $index=>$item)
        {
        if (stristr($item,$str))
        {
            return ($index+1);
            break;
        }
        }
        return -1;
    }

    /**
     * Check if str is contained in any of array's elements
     * 
     * @param type $str
     * @param type $arr
     * @return boolean 
     */

    function strInArray($str,$arr)
    {

        foreach ($arr as $item) 
        {
            if (self::inString($str,$item))
                return true;

        }

        return false;
    }
    }
}
?>

UPDATE :

In my controller, after loading the helper ($this->load->helper('dk/string');), I tried :

if (class_exists('dkString')) { echo "IT EXISTS<br/>Methods : "; print_r(get_class_methods('dkString')); }
else echo "IT DOESN'T EXIST";

The "funny" thing is that the output was :

IT EXISTS
Methods : Array ( 
[0] => characterCount 
[1] => beginsWith 
[2] => endsWith 
[3] => inString 
[4] => inStringCount 
[5] => strtonum 
[6] => indexInArray )

In a few words : the class IS loaded, and it "sees" ALL methods (except for the last one). Geezz... :/