Sometimes it requires to auto-populate data on the element based on the selection on another element e.g. City names based on a state.
You can do this with the only PHP but it required you to submit every time on selection.
This solves the problem but it is a little frustrating because it submits every time even if the selection is right or wrong.
For making it better you can use AJAX with jQuery that loads new data and remove the old one on each selection.
In the demonstration, I am creating a Department drop-down list and based on the option selection show all existing users of that department on another Dropdown.

1. Table structure

I am using 2 tables in the example –

department Table –

CREATE TABLE `department` (
  `id` int(11) NOT NULL PRIMARY KEY AUTO_INCREMENT,
  `depart_name` varchar(80) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

users Table –

CREATE TABLE `users` (
  `id` int(11) NOT NULL PRIMARY KEY AUTO_INCREMENT,
  `username` varchar(80) NOT NULL,
  `name` varchar(80) NOT NULL,
  `email` varchar(80) NOT NULL,
  `department` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

2. Configuration

Create a new config.php to define database connection.

<?php
$host = "localhost"; /* Host name */
$user = "root"; /* User */
$password = ""; /* Password */
$dbname = "tutorial"; /* Database name */

$con = mysqli_connect($host, $user, $password,$dbname);
// Check connection
if (!$con) {
  die("Connection failed: " . mysqli_connect_error());
}?>

3. HTML

Creating two Dropdown elements one –

• Fetch records from department table and use to add <option> in <select id='sel_depart'>  and
• Another dropdown shows the users names which are filled with jQuery based on the department name selection from the first dropdown element.

<?php 
include "config.php";
?>

<div>Departments </div>
       
<select id="sel_depart">
   <option value="0">- Select -</option>
   <?php 
   // Fetch Department
   $sql_department = "SELECT * FROM department";
   $department_data = mysqli_query($con,$sql_department);
   while($row = mysqli_fetch_assoc($department_data) ){
      $departid = $row['id'];
      $depart_name = $row['depart_name'];
      
      // Option
      echo "<option value='".$departid."' >".$depart_name."</option>";
   }
   ?>
</select>
<div class="clear"></div>

<div>Users </div>
<select id="sel_user">
   <option value="0">- Select -</option>
</select>

4. PHP

Create a new getUsers.php file.
Fetching users based on department selection from the users table.
Initialize $users_arr Array with userid and name.
Return $users_arr Array in JSON format.

include "config.php";

$departid = $_POST['depart'];   // department id

$sql = "SELECT id,name FROM users WHERE department=".$departid;

$result = mysqli_query($con,$sql);

$users_arr = array();

while( $row = mysqli_fetch_array($result) ){
    $userid = $row['id'];
    $name = $row['name'];

    $users_arr[] = array("id" => $userid, "name" => $name);
}

// encoding array to json format
echo json_encode($users_arr);

5. jQuery

Sending AJAX request when an option selected from the first drop-down. Pass the selected option value as data and on successfully callback fill <select id='sel_user'> with response.

$(document).ready(function(){

    $("#sel_depart").change(function(){
        var deptid = $(this).val();

        $.ajax({
            url: 'getUsers.php',
            type: 'post',
            data: {depart:deptid},
            dataType: 'json',
            success:function(response){

                var len = response.length;

                $("#sel_user").empty();
                for( var i = 0; i<len; i++){
                    var id = response[i]['id'];
                    var name = response[i]['name'];
                    
                    $("#sel_user").append("<option value='"+id+"'>"+name+"</option>");

                }
            }
        });
    });

});

6. Conclusion

In this tutorial, I used dropdown element for autofill you can do this with any other element like Textbox, Textarea, etc. that you can fill with information according to input on another element.