How to get JSON data from JavaScript to PHP

Ajax Course

This tutorial explains with code examples how to get JSON data sent with Ajax from JavaScript to PHP.
You can use one of these variants:

Using the php://input stream

- Documentation about PHP I/O streams
In this case the JSON data is sent as string via ajax with application/json Content-type.
- Example.
In JavaScript:

<script>
var data ={s1:'coursesweb.net', s2:'gamv.eu', y:2020};

//jsn_str = json string
function ajaxF(jsn_str) {
 var request = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP'); // XMLHttpRequest object

 request.open('POST', 'test.php', true); // set the request

 //sends data as json
 request.setRequestHeader('Content-type', 'application/json');
 request.send(jsn_str);

 // Check request status
 // If the response is received completely, alert response
 request.onreadystatechange =()=>{
 if(request.readyState ==4){
 alert(request.responseText); // coursesweb.net
 }
 }
}

//converts data object in json string and sends it to php
let jsn = JSON.stringify(data);
ajaxF(jsn);
</script>

In PHP (test.php):

$arr = json_decode(file_get_contents('php://input'), true);
echo $arr['s1'];
exit;

Adding JSON data in the $_POST variable

In this case the JSON data is asigned to a name (here "jsn"), and is sent as string via ajax with POST and application/x-www-form-urlencoded Content-type.
- Example.
In JavaScript:

<script>
var data ={s1:'coursesweb.net', s2:'gamv.eu', y:2020};

//jsn_str = json string
function ajaxF(jsn_str) {
 var request = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP'); // XMLHttpRequest object

 request.open('POST', 'test.php', true); // set the request

 //sends data
 request.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
 request.send('jsn='+jsn_str);

 // Check request status
 // If the response is received completely, alert response
 request.onreadystatechange =()=>{
 if(request.readyState ==4){
 alert(request.responseText); // gamv.eu
 }
 }
}

//converts data object in json string and sends it to php
let jsn = JSON.stringify(data);
ajaxF(jsn);
</script>

In PHP (test.php):

$arr = isset($_POST['jsn']) ? json_decode($_POST['jsn'], true) :['s2'=>'default'];
echo $arr['s2'];
exit;

<img src="http://coursesweb.net/imgs/webcourses.gif" width="191" height="63" alt="Courses-Web" />

#id {
  display: none;
}

for(var i = 0; i< 8; i++) {
  if(i > 1) break;
  alert(i);
}

define("CONSTANT_NAME", "value");
echo CONSTANT_NAME;