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;

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which tag adds an image in web page?
<div> <img> <span>
<img src="http://coursesweb.net/imgs/webcourses.gif" width="191" height="63" alt="Courses-Web" />
Which of these CSS codes displays the text oblique?
font-style: italic; text-decoration: underline; font-weight: 500;
#id {
  font-style: italic;
}
Click on the jQuery function used to hide with animation a HTML element.
click() hide() show()
$(document).ready(function() {
  $(".a_class").click(function(){ $(this).hide("slow"); });
});
Click on the correctly defined function in PHP.
fname function() {} function fname() {} function $fname() {};
function fname($a, $b) {
  echo $a * $b;
}
How to get JSON data from JavaScript to PHP

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