index.php

<?php

$con = mysqli_connect('localhost','root','');

mysqli_select_db($con, 'final-group-chat');

if(isset($_POST['insertUser']))
{
	if($_POST['username'] != "")
	{
		$userName = $_POST['username'];
		
		$query = "SELECT * FROM user WHERE user_name = '".$userName."'";

		$result = mysqli_query($con, $query);

		if(mysqli_num_rows($result)>0)
		{
			header('Location: groups.php?name='.$userName);			
		}
		else
		{	
			$query1 = "INSERT INTO `user` (`user_name`) VALUES ('$userName')";

			if(!mysqli_query($con,$query1)){
				echo "Error: Unable to Insert NewUser";
			}
			else{
				echo "User Entered successfully";
			}
		}
	}
	else
	{
	echo "Error: Enter Name";
	}

}

?>

<!DOCTYPE html>
<html>
<head>
	<title>USER</title>
	
</head>
<body>

<form action="index.php" method="POST">
	<label for="username">Enter Your Name:</label></br></br>
	<input type="text" name="username" id="username"></br></br>
	<button name="insertUser" type="submit">Submit</button>
</form>

</body>
</html>