PHP Random String + StringArray Generator

<?php

class Tools {

    /**
     * Generates a random string of a variable length and charset
     * @param int $length
     * Length of the returned string
     * @param string $chars
     * Charset
     * @return string
     * Random string of the set length using the set charset
     */

    public static function GenerateRandomString($length = 10, $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890') {
        $cCount = strlen($chars);
        $r = '';
        for ($i = 0; $i < $length; $i++) {
            $r .= $chars[rand(0, $cCount - 1)];
        }
        return $r;
    }


    /**
     * Generates an array of random strings (no doubles) with a variable count, length and charset
     * @param $count
     * Amount of entries in the returned array
     * @param $length
     * Length of the returned strings
     * @param string $chars
     * Charset
     * @return array|bool
     * Array on success - False on failure (requested a higher count than possible with the set charset and string length)
     */
    public static function GenerateRandomStringArray($count, $length, $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890') {
        if(pow(strlen($chars), $length) >= $count){
            $arr = array();
            $i = 0;
            while($i < $count) {
                $str = Tools::GenerateRandomString($length, $chars);
                if(!in_array($str, $arr)) {
                    $arr[] = $str;
                    $i++;
                }
            }
            return $arr;
        }
        else return false;
    }
}

?>

Tools::GenerateRandomStringArray(10, 16) =>
array(10) {
    [0]=> string(16) "eCegsdzjJqhjxky9"
    [1]=> string(16) "Qv9686e3lCykXekM"
    [2]=> string(16) "WvgLLwB7pU3akJOM"
    [3]=> string(16) "vAChnzHwXhKRGO18"
    [4]=> string(16) "wLcDXdvhXLGoZvWN"
    [5]=> string(16) "23YQwr3orkPLhMzO"
    [6]=> string(16) "ZoZT3giPoZcZxEWb"
    [7]=> string(16) "qRdMwGgCOKM4K81T"
    [8]=> string(16) "jpEwYHWLHBAXWbOX"
    [9]=> string(16) "G4k7pjle4xCH7Pkm"
}

Tools::GenerateRandomStringArray(10, 16, "01") =>
array(10) {
    [0]=> string(16) "0001001010011111"
    [1]=> string(16) "0001110100010100"
    [2]=> string(16) "0000110011001110"
    [3]=> string(16) "0100010100100011"
    [4]=> string(16) "0001110010010101"
    [5]=> string(16) "1001000111011110"
    [6]=> string(16) "1000111000111110"
    [7]=> string(16) "0111110010111100"
    [8]=> string(16) "1100010100100010"
    [9]=> string(16) "1100000011010001"
}