php生成随机数类

<?php
class getRandstrClass{
    function getCode ($length = 32, $mode = 0) {
        switch ($mode) {
        case '1':
            $str = '1234567890';
            break;
        case '2':
            $str = 'abcdefghijklmnopqrstuvwxyz';
            break;
        case '3':
            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
            break;
        case '4':
            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
            break;
        case '5':
            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890';
            break;
        case '6':
            $str = 'abcdefghijklmnopqrstuvwxyz1234567890';
            break;
        default:
        $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890';
        break;
        }
        $randString = '';
        $len = strlen($str)-1;
        for($i = 0;$i < $length;$i ++){
            $num = mt_rand(0, $len);
            $randString .= $str[$num];
        }
        return $randString ;
    }
}
/* 使用方法 
$code = new getRandstrClass(); 
$length = 4; 
$mode = 0; 
$str = $code->getCode($length, $mode); 
echo $str;
$code = NULL;
*/
?>