Why is my PHP server returning my HTML code?

[DumbestPerson224]

Body

let data = new FormData();
let xhttp = new XMLHttpRequest();
function update(){
    data.append("auto_update", document.getElementById("auto_update").value);
    xhttp.onreadystatechange = function(){
      if(this.readyState == 4 && this.status == 200){
        let response = document.createElement("p");
        response.innerText = this.responseText;
        document.body.appendChild(response);
      }
    } 
    xhttp.open("GET", "upload.php");
    xhttp.send(data);
}

Guidelines

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[satulg01]

Please, post your PHP code.