PHP生成一串随机字母和数字

<?php
class GetChar{

function getCode ($length = 32, $mode = 0){
    switch ($mode) {
    case '1':
    $str = '1234567890';
    break;
    case '2':
    $str = 'abcdefghijklmnopqrstuvwxyz';
    break;
    case '3':
    $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    break;
    default:
    $str = '1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    break;
    }

    $result = '';
    $l = strlen($str)-1;
    $num=0;

    for($i = 0;$i < $length;$i ++){
        $num = rand(0, $l);
        $a=$str[$num];
        $result =$result.$a;
    }
return $result;
}
}

$code = new GetChar;
echo $code->getCode();
?>