//Insert User from form - Prepared statements with placeholders
<?php
//INSERT USER
    if(isset($_POST['submit'])){
        $uname = trim($_POST['username']);
        $email = trim($_POST['email']);
        $pwd = "secret";

        if(empty($uname) || empty($email)){
            $error = true;
        }else{
          $sql = 'INSERT INTO users SET user_name = ?, user_email = ?, user_password = ?';
          $stmt = mysqli_stmt_init($link);
          if(!mysqli_stmt_prepare($stmt, $sql)){
            die('Query failed');
          }else{
            mysqli_stmt_bind_param($stmt, 'sss', $uname,$email,$pwd);
            mysqli_stmt_execute($stmt);
          }
        }
    }
?>

//Form
<form class="py-4" action="index.php" method="post">
   <div class="row">
       <div class="col-md-4">
          <input type="text" name="username" class="form-control" placeholder="Username" aria-label="Username">
        </div>
        <div class="col-md-4">
          <input type="email" name="email" class="form-control" placeholder="Email Address" aria-label="Email Address">
        </div>
        <div class="d-grid gap-2 col-4 mx-auto">
          <input type="submit" name="submit" class="form-control btn btn-secondary" value="Add New User">
          <?php echo isset($error) ? "<p>Field can't be blank</p>": ''; ?>
        </div>
    </div>//End of row
</form>