如何使用POST使用vanilla JavaScript将变量值​​传递到php文件？

I have created an object which has the values of input elements in form. I have 3 pages in the form and i'm getting those pages using XMLHttpRequest. I need to send all those values to a php file and store them in variables of php file.Then i can insert them into the database.

I need to send these variables using XMLHttpRequest and POST method. So I used this code.

// obj1 is the object which stores the values of input fields


var xhr = new XMLHttpRequest();
 xhr.onreadystatechange = function () {
  if(xhr.readyState == 4 && xhr.status == 200 ) {
   document.getElementById("newForm").innerHTML = xhr.responseText;
}
};

 xhr.open("POST","Post Data.php", true);
 xhr.setRequestHeader("Content-type", "multipart/form-data");
 xhr.send("propertyX=obj1.property1");
}

It inserts nothing into the database.

This is the part of php file

$time = time();
$time = date("Y-m-d", $time);
$propertyX = $_POST["property1"];

Can anybody please help me to fix this?