How to match the SESSION username with the written username?

So I’m creating this test Website and I’m trying to make an edit profile page by user and I want to make the username unchangeble and the user is only able to edit thier profile if they type in thier username in editprofileHTML.php I have this input:

<input type="text" class="form-control" id="username" name="username" value="<?php echo  $username; ?>">

I tried to make it disabled but it didn’t work.

editprofile.inc.php:

 $updatedData = [
    "firstname" => $_POST["firstname"],
    "lastname" => $_POST["lastname"],
    "phoneNr" => $_POST["phoneNr"],
    "username" => $_POST["username"],
    "gender" => $_POST["gender"],
];

if (matchingUsername($updatedData["username"]) === true) {
    $error = '<p class="error" > Please write your Username correctly </p>';
}
updateUser($updatedData);

$_SESSION["loggedInUser"] = loadUserByUsername($_POST["username"]);
header('Location: ../profile.php');
}

And I also have these functions:

function updateUser($updatedData) {
    $dataToUpdate = [];
    foreach ($updatedData as $key => $value) {
        if ($updatedData[$key] !== "" ) {
            $dataToUpdate[$key] = $value;
        }
    }
if (!empty($dataToUpdate)) {
    $sql = 'UPDATE login SET ';
    $arrayLength = count($dataToUpdate);
    $i = 0;
    foreach ($dataToUpdate as $key => $value) {
        $sql .= $key . ' = "'.$value.'"';
        if (++$i !== $arrayLength) {
            $sql .= ', ';
        }
    }
    $sql .= ' WHERE username = "'.$updatedData["username"].'"';
    global $conn;
    $conn->query($sql);

    if ($conn->error){
        throw new Exception("Error updating user: " . $conn->error);
    }
}

function matchingUsername($username) {
    $user = loadUserByUsername($username);
    if ($username === $user) {
        return true;
    } return false;
}
function invalidUsername($username) {
    $user = loadUserByUsername($username);
    if (!empty($user)) {
        return true;
    }
}
function loadUserByUsername($username) {
global $conn;
$sql = 'SELECT * FROM login WHERE username = "'.$username.'"';
$result = $conn->query($sql);

return $result->fetch_assoc();
}