What's wrong with this code?

Question

trying to update data<?php

include("connect.php");
include("functions.php");

if(logged_in())
{
?>

<a href="changepassword.php">Change Password</a>
<a href="logout.php" style="float:right; padding:10px; margin-right:40px; background-color:#eee; color:#333; text-decoration:none;">Logout</a>

<?php

}
else
{
header("location:login.php");
exit();
}

$amount = "";
$id="";
$error="";
if(isset($_POST['submit']))
{
$userid = $_POST['id'];
$amount = $_POST['amount'];
$verify = mysqli_query($con,"SELECT amount FROM users WHERE id = '$id'");

$row=mysqli_num_rows($verify);
if ($row <= 0) {
$error = "invalid username or password!";
}else{
$sql = "UPDATE users SET amount='.$amount.' WHERE id='$id'";
mysqli_query($con,$sql);
}
}
?>
This is form code
<div id="formDiv">

<form method="POST" action="profile.php" enctype="multipart/form-data">

<label>ID:</label><br/>
<input type="text" name="id" class="inputFields" required/><br/><br/>

<label>Amount</label><br/>
<input type="text" name="amount" class="inputFields" required/><br/><br/>

<input type="submit" class="theButtons" name="submit"/>

</form>

Answer 1

Also, syntax error.
The right syntax is:
else {
$sql ="UPDATE users SET amount = '" . $amount . "' WHERE id = '" . $id . "'";
//Blocks of code
}

Answer 2

Headers must be ALWAYS before any other page content. Rebuild the if-else condition.
Also, does your 'logged_in()' function return bool datatype?