get url parameter using javascript and send it to php

Solution:

var data = "id="+document.getElementById("id").value;
                                         ^^^^--string containing the letters 'i' and 'd'
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should probably be

var data = "id="+document.getElementById(id).value;
                                         ^^-- variable named "id"
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Solution:

try this function

function getParameterByName( name ) {
  var parts = window.location.search.substr(1).split("&");
  var p_arr = {};
  for( var i = 0; i < parts.length; i++ ) {
    var temp = parts[i].split("=");
    if ( decodeURIComponent( temp[0] ) != "" ) {
      p_arr[decodeURIComponent( temp[0] )]  = decodeURIComponent( temp[1] );
    }
  }
  return p_arr[name];
}

function getResults() {
  var xmlhttp;
  if ( typeof XMLHttpRequest !== 'undefined' ) {
    xmlhttp = new XMLHttpRequest();
  }
  else {
    var versions  = [ "MSXML2.XmlHttp.5.0", "MSXML2.XmlHttp.4.0", "MSXML2.XmlHttp.3.0", "MSXML2.XmlHttp.2.0", "Microsoft.XmlHttp" ];
    for( var i = 0, len = versions.length; i < len; i++ ) {
      try {
        xmlhttp = new ActiveXObject( versions[i] );
        break;
      }
      catch(e) {}
    }
  }

  xmlhttp.onreadystatechange  = function() {
    if ( this.readyState === 4 && this.status == 200 ) {
      document.getElementById( "here" ).innerHTML = xmlhttp.responseText;
    }
  };

  var id    = getParameterByName( "id" );
  var data  = "id="+document.getElementById( id ).value;

  xmlhttp.open( "POST", "add.php", true );
  xmlhttp.setRequestHeader( "Content-Type", "application/x-www-form-urlencoded" );
  xmlhttp.send( data );
}
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