<select name="icerik" id="secenek">
  <option value="1">Seçenek 1</option>
  <option value="2">Seçenek 2</option>
  <option value="3">Seçenek 3</option>
</select>
<div id="gelecek"></div>
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script>
  $(document).ready(function()
{
    $('#secenek').change(function()
    {
        var value = $('#secenek option:selected').attr('value');
        $.ajax({url: "secenekgetir.php?secenek="+value, success: function(result){
        $("#gelecek").html(result);
        }});
    })
})
</script>

<select name="kategori">
<?php
require_once "baglan.php";
$secenek=$_GET["secenek"];
$sorgu=mysql_query("select * from secenekler where secenek=".$secenek);
if(mysql_affected_rows() > 0)
{
while($oku=mysql_fetch_assoc($sorgu))
{
echo '
 <option value="'.$oku["kategoriID"].">'.$oku["kategoriAdi"].'</option>';
}
}
?>
</select>