jquery part
$('#registration_form').on('submit',function(e){
    e.preventDefault();
    var datastring = $(this).serialize();
    $.ajax({
       method : "GET",
       url : "<?php echo base_url(); ?>Main/submit_registration_form",
       data: {datastring:datastring}
    })
    .done(function(msg){
       alert(msg);
    });
  });

  php part

  public function submit_registration_form(){
        parse_str($_GET['datastring'], $searcharray);
        echo $searcharray['date_of_birth'];
    }