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 is used to add lists into <ul> and <ol> elements?
<dt> <dd> <li>
<ul>
 <li>http://coursesweb.net/html/</li>
 <li>http://coursesweb.net/css/</li>
</ul>
Which value of the "display" property creates a block box for the content and ads a bullet marker?
block list-item inline-block
.some_class {
  display: list-item;
}
Which instruction converts a JavaScript object into a JSON string.
JSON.parse() JSON.stringify eval()
var obj = {
 "courses": ["php", "javascript", "ajax"]
};
var jsonstr = JSON.stringify(obj);
alert(jsonstr);    // {"courses":["php","javascript","ajax"]}
Indicate the PHP class used to work with HTML and XML content in PHP.
stdClass PDO DOMDocument
$strhtml = '<body><div id="dv1">CoursesWeb.net</div></body>';
$dochtml = new DOMDocument();
$dochtml->loadHTML($strhtml);
$elm = $dochtml->getElementById("dv1");
echo $elm->nodeValue;    // CoursesWeb.net
How to get JSON data from JavaScript to PHP

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