无法使用XMLHttpRequest将参数传递给$_POST

var xhr = new XMLHttpRequest();
var formData = new FormData();

formData.append('param1', 'value1');
formData.append('param2', 'value2');

xhr.open('POST', 'server-url', true);
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    // 请求成功处理
  }
};
xhr.send(formData);

var xhr = new XMLHttpRequest();

xhr.open('POST', 'server-url', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    // 请求成功处理
  }
};

var params = 'param1=value1&param2=value2';
xhr.send(params);