serializers - ajax传递过去的data是一个序列化的字符串，为什么php接受过去确实一个数组?

前端代码：
if(register_flag){ //注册信息都正确

        //alert(typeof($('.register-form').serialize()));
        //序列化的结果:name=hesisi&account=15223306809&verification-code=2333&password=hss123456&confirm-pwd=hss123456
        $.ajax({
            type : 'POST',
            url : 'php/register.php',
            data : $('.register-form').serialize(),  //序列化的字符串
            success : function(data){
                //window.location.href = "index.html";
            },
            error : function(){
                //错误信息处理
                console.log();
            }

        });            

    }

php代码
require 'config.php';

$data =$_POST;
//name=hesisi&account=15223306809&verification-code=2333&password=hss123456&confirm-pwd=hss123456
$arr = explode("&",$data);
$name_arr = explode("=",$arr[0]);
$account_arr = explode("=",$arr[1]);
$verifcode_arr = explode("=",$arr[2]);
$password_arr = explode("=",$arr[3]);
$confirmpwd_arr = explode("=",$arr[4]);

$name = $name_arr[1];
$account = $account_arr[1];
$verifcode = $verifcode_arr[1];
$password = $password_arr[1];
$confirmpwd = $confirmpwd_arr[1];

$mobile_code = $_SESSION['mobile_code'];

$query = "SELECT * from user WHERE user_account=".$account;
$result = mysqli_query($query);

if($verifcode != $mobile_code){//手机验证码错误
    exit("手机验证码错误！");
    return;
}else if($result){
    exit("改手机号已经注册！");
    return;
}else{
    $insert = "INSERT INTO user(user_name,password,user_account) VALUES(".$name.",".$password.",".$account.")";
    mysqli_query($insert);
    exit("注册成功！");
}

这里报错说，explode()第二个参数应该是string类型，但是我穿的是数组类型。ajax传过去的data是string类型啊，为什么php通过$_POST[]接受就是array类型了？data之前没写过php，请指教，谢谢~