How to check for duplicate username or email id PHP MySQLi

In this example i am going to explain how to check for duplicate username or email id PHP MySQLi.

connect.php

<?php
$url='localhost';
$username='root';
$password='';
$conn=mysqli_connect($url,$username,$password,"crud");
if(!$conn){
 die('Could not Connect My Sql:' .mysql_error());
}
?>

index.php

<!DOCTYPE html>
<html>
<body>

<form method="post" action="process.php">
  First name:<br>
  <input type="text" name="user_name" >
  <br>
  Last name:<br>
  <input type="text" name="email_id">
  <br><br>
  Password:<br>
  <input type="password" name="password">
  <br><br>
  <input type="submit" value="Submit">
</form> 



</body>
</html>

process.php

<?php
include('connect.php');
if(isset($_POST['save']))
{
$user_name = $_POST['user_name'];
$email_id = $_POST['email_id'];
$user_password = $_POST['password'];
$duplicate=mysqli_query($conn,"select * from user_login where user_name='$user_name' or email_id='$email_id'");
if (mysqli_num_rows($duplicate)>0)
{
header("Location: index.php?message=User name or Email id already exists.");
}
else{
try {
date_default_timezone_set("Asia/Calcutta");
$insertdate = date("Y-m-d H:i:s");
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$sql = "INSERT INTO user_login (email_id,user_name,password,date_time)
VALUES ('$email_id', '$user_name','$user_password','$insertdate')";
$conn->exec($sql);
header("Location: index.php?message=Thank you for register. Please login to continue.");
}
   catch(PDOException $e)
    {
          echo $sql . "
          " . $e->getMessage();
    }
$conn = null;
}
}
?>

Demo

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