When a new user  registered for the first time the account must verified. It can be done in different ways. 

// REGISTER USER
if (isset($_POST['register_btn'])) {
// receive all input values from the form
$firstname = esc($_POST['firstname']);
$lastname = esc($_POST['lastname']);
$genders=esc($_POST["gender"]);
$username = esc($_POST['username']);
$email = esc($_POST['email']);
$password_1 = esc($_POST['password_1']);
$password_2 = esc($_POST['password_2']);
// form validation: ensure that the form is correctly filled
if (empty($firstname)) {  array_push($errors, "Uhmm...You missed your First Name"); }
if (empty($lastname)) {  array_push($errors, "Uhmm...You missed your Last Name"); }
if (empty($username)) {  array_push($errors, "Uhmm...We gonna need your username"); }
if (empty($email)) { array_push($errors, "Oops.. Email is missing");}                                    
if (empty($password_1)) { array_push($errors, "uh-oh you forgot the password"); }
if ($password_1 != $password_2) { array_push($errors, "The two passwords do not match");}
// Ensure that no user is registered twice. 
// the email and usernames should be unique
$user_check_query = "SELECT * FROM users WHERE username='$username' OR email='$email' LIMIT 1";
$result = mysqli_query($conn, $user_check_query);
$user = mysqli_fetch_assoc($result);    
		if ($user) { // if user exists
			if ($user['username'] === $username) {
			  array_push($errors, "Username already exists");
			}
			if ($user['email'] === $email) {
			  array_push($errors, "Email already exists");
			}
		}
		// register user if there are no errors in the form
		if (count($errors) == 0) {
			$password = md5($password_1);//encrypt the password before saving in the database
			$vkey = md5(time().$username); 
			$cenvertedTime =date('Y-m-d', strtotime(' + 5 days'));
			$query = "INSERT INTO users (firstname, lastname, gender, username, email, password, vkey, token_expire, created_at, updated_at) 
					  VALUES('$firstname', '$lastname', '$genders', '$username', '$email', '$password', '$vkey', '$cenvertedTime', now(), now())";
			$resultset = mysqli_query($conn, $query);
			
			$inserted_id = mysqli_insert_id($conn);
			$user_check = "SELECT token_expire FROM users WHERE id='$inserted_id' LIMIT 1";
            $result = mysqli_query($conn, $user_check);
		    $date = mysqli_fetch_assoc($result);
            $created_at = $date['token_expire'];
			$created_at = date("F j, Y ", strtotime($created_at));
			
        if($resultset){
        $auto = date (Y);
		$to=$email;
		$sent_date = date("F j, Y ");
		$subject='Please confirm your email address';
		$message="

let's get thingis together, finally send then email

	$message="include your custom body design/ I included message body in previous post";
		$headers = "MIME-Version: 1.0" . "\r\n";
        $headers .= "Content-type:text/html;charset=UTF-8" . "\r\n";
        $headers .= 'From: Do-Not-Reply noreply@g3techdesign.com'. "\r\n";
         
		mail($to, $subject, $message, $headers);
        
		$_SESSION['message'] = " Thank you for signing up in to our system. We have sent a verification email to the address you provide.
". $to . " is the email address you provide to us to activate your account.";
		
     	// redirect to public area
		header('location: '. BASE_URL . 'thankyou.php?email=' . $email);				
		exit(0);			
			}
			 else {
				$_SESSION['message'] = "Something want wrong.";
				// redirect to public area
				header('location: '. BASE_URL . 'signup.php');				
				exit(0);
			}
    }
}

Remove empty space from the form

// escape value from form
	function esc(String $value)
	{	
		// bring the global db connect object into function
		global $conn;

		$val = trim($value); // remove empty space sorrounding string
		$val = mysqli_real_escape_string($conn, $value);

		return $val;
	}

Then when a user click on the link they have recieved the account will activate  then they can able to login.

$errors = array(); 
if(isset($_GET['vkey'])){
	$vkey = $_GET['vkey'];
	$user_check = "SELECT verified,vkey, token_expire FROM users WHERE verified=0 AND vkey = '$vkey' LIMIT 1";
        $result = mysqli_query($conn, $user_check);
        $get_token = mysqli_fetch_assoc($result);
	  if($result->num_rows == 1){
	      $currentDateTime = date('Y-m-d H:i:s');
	    $token_date =$get_token['token_expire'];
	  if($currentDateTime < $token_date){
	  $sql = "UPDATE users SET verified=1 WHERE vkey='$vkey' LIMIT 1";
          $results = mysqli_query($conn, $sql);
	  $_SESSION['message'] = " Your Acount has been verified. You may now login in to your Account.";
      header('location: ' . BASE_URL . 'login.php');
      exit(0);
		}else{
		array_push($errors, " Verification link has been expired. You may attempt to create your account again. The account you created was deleted automatically");
		}
	  }else{
		array_push($errors, " This Account is already verified or invalid. You may try to login now.");
		}
}

that's.