file_put_contents("/tmp/test","xxxx")

<form id="myForm">
    <input type="text" name="name">
    <input type="text" name="addr">
    <input type="button" value="submit" id="submit">
</form>
<p id="result"></p>
<script>
function show(){
    var formData = new FormData( document.querySelector("#myForm") );
    var query_str = "ajax_get.php?";
    for(var pair of formData.entries())
    {
        query_str += pair[0] + "=" + pair[1] + "&";
    }
    query_str = query_str.substr(0,query_str.length - 1);
    var obj = new XMLHttpRequest();
    obj.onreadystatechange = function(){
        if(obj.readyState == 4 &&  obj.status == 200){
            console.log(obj.responseText);
        }
    }
    obj.open("get",query_str);
    obj.send();
}

ob = document.getElementById("submit");
ob.addEventListener("click",show);
</script>

<?php
ini_set('display_errors', 1);
error_reporting(E_ALL);
$str = implode(" ",$_GET);
file_put_contents("/tmp/test",$str);
print($str);
?>

$handle=fopen("/tmp/test","w");
fwrite($handle,$str);
fclose($handle);