register.php

<script>
  function loadDoc() {
  var username = document.forms["form"]["reg_username"].value;
  var email = document.forms["form"]["reg_email"].value;
  var password = document.forms["form"]["reg_password"].value;
  var cpassword = document.forms["form"]["c_password"].value;
  var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (xhttp.readyState == 4 && xhttp.status == 200) {
     document.getElementById("demo").innerHTML = xhttp.responseText;
    }
  };
  xhttp.open("POST", "doregister.php", true);
  xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhttp.send("reg_username=" + username + "&reg_email=" + email + "&reg_password=" + password + "&c_password=" + cpassword);
}
</script>
  <form name='form'>
  <input type='text' name='reg_username' placeholder='Username'/>
  <input type='text' name='reg_email' placeholder='Email'/>
  <input type='password' name='reg_password' placeholder='Password'/>
  <input type='password' name='c_password' placeholder='Confirm Password'/>
  <button type='button' onclick='loadDoc()'>Register</button>
  <div id='demo'></div>
  </form>