how to pass a variable from javascript function to php file without reloading the current html page in which javascript function is written?

function change( el)
{
    if ( el.value == "ON" )
    { 
        el.value = "OFF"; 
        el.style.backgroundColor = "#ff3333"; /* Red */
    }
    else
    { 
        el.value = "ON"; 
        el.style.backgroundColor = "#4CAF50"; /* Green*/
    } // works till here like expected.



    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() 
    {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) 
        {
            }

    }
    xmlhttp.open("GET", "postform.php?switch=" + el.value, true)
    xmlhttp.send();
}