Category and subcategory in the same table

Talk the guys! I need to create a system that includes categories and subcategories ... however, these two must be in a single table with the columns id, category, category_id and exc. Does anyone know how I should do this? Thank you in advance!

    <?php


if (!empty($_POST['formCategoria'])) {

include '../fn.php';


if (empty($_POST['categoria_id'])) {
    s("INSERT INTO categoria (categoria_id) VALUES (NULL)");
    $f = f("SELECT categoria_id FROM categoria ORDER BY id DESC LIMIT 1");
    $_POST['id'] = $f['id'];
}
$categoria_id="";
if (!empty($_POST['categoria_id'])) {
    $categoria_id=" categoria_id='" . $_POST['categoria_id'] . "', ";
}

s("UPDATE categoria SET
    ".$categoria_id."
    categoria_id='" . $_POST['categoria_id'] . "'
        WHERE id='" . $_POST['id'] . "'
");
?>
<script>
    window.alert('Atualizado');
    window.location = '../categorias';
</script>
<?php
exit;
}

if (!empty($_GET['categora_id'])) {
$categoria = f("SELECT * FROM categoria WHERE id=" . $_GET['categoria_id']);
}
?>
<div class="row">
<div class="container">
    <div class="col-md-12">

        <h3>Subcategoria</h3>

        <a class="btn btn-xs btn-default" href="<?php echo $root; ?>categorias">Voltar</a>

        <div class="br2"></div>
        <div class="row">
            <form id="formCategoria" action="<?php echo $root; ?>pg/edita-subcategoria.php" method="post">
                <input type="hidden" name="formCategoria" value="ok">
                <?php
                if (!empty($_GET['id'])) {
                    ?>
                    <input type="hidden" name="id" value="<?php echo $_GET['id']; ?>">
                    <?php
                }
                ?>
                <div class="col-md-3">
                    <div class="form-group">
                        <label>Subcategoria</label>
                        <input class="form-control" type="text" name="categoria_id"
                               value="<?php echo!empty($categoria_id['categoria_id']) ? $categoria_id['categoria_id'] : ''; ?>">
                    </div>
                </div>

                <div class="br2"></div>
                <div class="col-md-12">
                    <div class="div-right">
                        <input type="submit" class="btn btn-default" value="<?php echo empty($_GET['id'])?'Cadastrar':'Atualizar'; ?>">
                    </div>
                </div>
                <div class="br2"></div>
                <div class="br2"></div>
            </form>
        </div>
    </div>
</div>