<form action="sec.php" method="post">
<select name="kategoriler[]" multiple>
<option value="1">Araba</option>
<option value="2">TV</option>
<option value="3">Bilgisayar</option>
</select>
<input type="submit">
</form>

sec.php
$kategoriler = $_POST['kategoriler'];
$alanlar= str_repeat('?, ', count($kategoriler) - 1) . '?';
$query = $db->prepare("SELECT * FROM urunler WHERE c_id IN ($alanlar)");
$query->execute($kategoriler);
$query = $query->fetchAll(PDO::FETCH_OBJ);
print_r($query);