Difference between GET and POST as well as Asynchronous and Synchronous Ajax / XMLHTTPRequest..

asynchsynch.php

<?php
Header("Content-Type:Text/Javascript");
$_requests = ["POST"=>[],"GET"=>[]];
$_requests["POST"]["ASYNCH"] = ("var xhr = new XMLHttpRequest();
xhr.open('POST','//".$_SERVER["HTTP_HOST"]."/handler',true);
xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
xhr.onreadystatechange = function(){
	if(xhr.readyState == 4 && xhr.status == 200){
		var result = xhr.responseText;
	}
}
xhr.send('data=hello');");
$_requests["POST"]["SYNCH"] = ("var xhr = new XMLHttpRequest();
xhr.open('POST','//".$_SERVER["HTTP_HOST"]."/handler',false);
xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
xhr.onreadystatechange = function(){
	if(xhr.readyState == 4 && xhr.status == 200){
		var result = xhr.responseText;
	}
}
xhr.send('data=hello');");
$_requests["GET"]["ASYNCH"] = ("var xhr = new XMLHttpRequest();
xhr.open('GET','//".$_SERVER["HTTP_HOST"]."/handler',true);
xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
xhr.onreadystatechange = function(){
	if(xhr.readyState == 4 && xhr.status == 200){
		var result = xhr.responseText;
	}
}
xhr.send(null);");
$_requests["GET"]["SYNCH"] = ("var xhr = new XMLHttpRequest();
xhr.open('GET','//".$_SERVER["HTTP_HOST"]."/handler',false);
xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
xhr.onreadystatechange = function(){
	if(xhr.readyState == 4 && xhr.status == 200){
		var result = xhr.responseText;
	}
}
xhr.send(null);");
print_r($_requests);
//True for Asynchronous
//False for Synchronous
?>