$kosul = '';

if($marka> ' '){ $kosul = ' and marka=\''.$marka.'\'' ; }
if($model > ' '){ $kosul = $kosul . ' and model=\''.$model.'\''; }
if($sahip > ' '){ $kosul = $kosul . ' and sahip=\''.$sahip.'\''; }
if($sehir > ' '){ $kosul = $kosul . ' and sehir=\''.$sehir.'\''; }

$sql='select * from ilan where 1 ' . $kosul;