使用javascript设置输入元素的值，并使用php获取该值

document.getElementById("inputElementId").value = "设置的值";

document.querySelector("input[name='inputElementName']").value = "设置的值";

<form action="process.php" method="post">
  <input type="text" name="inputElementName">
  <input type="submit" value="提交">
</form>

$inputValue = $_POST['inputElementName'];

var inputValue = document.getElementById("inputElementId").value;
var xhr = new XMLHttpRequest();
xhr.open("POST", "process.php", true);
xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    // 处理服务器端返回的响应
  }
};
xhr.send("inputElementName=" + encodeURIComponent(inputValue));

$inputValue = $_POST['inputElementName'];