make random strings to use php

RandStr.php

<?php

class RandStr{

    private $small = 'abcdefghjiklmnopqrstuvwxyz';

    private $large = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

    private $num = '0123456789';

    private $all_ary = array();

    function __construct(){
        array_push($this->all_ary,$this->small,$this->large,$this->num);
    }

    public function add_str($param = array()){
        foreach($param as $key => $value){
            $this->$key = $value;
            $this->all_ary[] = $value;
        }
    }

    public function mk_rd_str($length,$param = array('small','large','num')) {
        $carset = '';

        foreach($param as $val){
            $charset .= $this->$val;
        }

        $charset_length = strlen($charset);

        $string = '';

        for ($i = 0; $i < $length; $i++) {
            $string .= $charset[mt_rand(0, $charset_length - 1)];
        }

        return $string;
    }

}