In PHP use this to get the string sent with ajax :
$data = file_get_contents("php://input");
And in JS :
dataString = txCryptData;
var xhr = new XMLHttpRequest();
var params="data="+dataString;
xhr.onreadystatechange = function() {
if (xhr.readyState == 4 && xhr.status==200) {
alert(xhr.responseText);
}
}
xhr.open('POST', 'tokenize.php', true);
xhr.send(params);