為什么js二維數(shù)組傳給后臺接收到的數(shù)據(jù)變成了字符串

問題：前端傳給后臺的數(shù)據(jù)是[["aa","2",3],["bb","3",4]]二維數(shù)組，
PHP后臺收到的卻是字符串

let arr = [["ll", 4], ["bb", 3]];

let uid = sessionStorage.getItem('uid');
this.$refs[formName].validate((valid) => {
    if (valid) {
        var params = new URLSearchParams();
        params.append('is_iso', '1');
        params.append('uid', uid);
        params.append('order',arr);
        this.$axios({
            method: 'post',
            url:httpUrl.add_investment_project,
            data:params,
        }).then((res)=>{
            console.log(res.data);
            if(res.data.errCode==0){
            
            }else if(res.data.errCode==1){
                this.$message.error(res.data.retData.msg);
            }else if(res.data.errCode==2){
                this.$router.push('/login');
            }
        });
    } else {
        this.$message.error('error submit!!');
        return false;
    }
});