login

<?php

function showDataTable($query)
{
    global $conn;
    $resultShowDataTable = mysqli_query($conn, $query);
    $wrapData = [];

    while ($row = mysqli_fetch_assoc($resultShowDataTable)) {
        $wrapData[] = $row;
    }

    return $wrapData;
}

//udah bener neh.. pake isset.. gunakan pada isset dibawah
if (isset($_POST["check-account"])) {
    $username = $_POST["username"];
//tidak tepat
//    $checkUsername = showDataTable("SELECT * FROM login_siswa54 WHERE username = '$username'")[0];
    $checkUsername = showDataTable("SELECT * FROM login_siswa54 WHERE username = '$username'");
    if ($username === '') {
        echo "<script>alert('Lengkapi kolom input terlebih dahulu!')</script>";
    } else {
        if (isset($checkUsername[0])) //$checkUsername === NULL)
        {
            $usernameError = true;
        } else {
            $idGET = $checkUsername["id"];
            header("Location: forget-password.php?id=$idGET");
        }
    }
}
?>

baris error ada di
$checkUsername = showDataTable("SELECT * FROM login_siswa54 WHERE username = '$username'")[0];