การเชื่่อม DB และกำหนดเงื่อนไข login

**connectdb.php
<?php
    $servername ="localhost";
    $username = "root";
    $password = "12345678";
    $dbname = "abc";
      
    $conn = mysqli_connect($servername,$username,$password,$dbname) or die("Cannot connect mysql");
    //mysqli_select_db($dbname,$link) or die("Cannot select $dbname");
    mysqli_set_charset($connection, "utf8")
?>

**index.php
<?session_start();
include("connectdb.php");
/*$sql="select * from logist";*/

$result = mysql_query ($sql);
while ( $row= mysql_fetch_array ( $result ) ){
	$username=$row["username"];
	$password=$row["password"];
	
    echo $username,$password;
    echo "<br>";
}
?>
<div class="form-group">
                                <label for="username" class="text-info">Username</label><br>
                                <input type="text" name="username" id="username" class="form-control">
                            </div>
                            <div class="form-group">
                                <label for="password" class="text-info">Password</label><br>
                                <input type="password" name="password" id="password" class="form-control">
                            </div>
                            <div class="form-group">
                                <!------<input type="submit" name="submit" class="btn btn-info btn-md "value="Login"> ---------->
                                <a href="main.php" name="login" class="u-btn u-button-style u-palette-2-base u-btn-1">Log in</a>
                                
                            </div>