[مشكلة] نتائج صحيحة واحيانا خاطئة

رمز PHP:

$a1 = $_GET['txt_type'];
$a2 = $_GET['choose1'];
$a3 = $_GET['choose2'];
$a4 = $_GET['price1'];
$a5 = $_GET['price2'];
@extract(mysql_fetch_array(mysql_query("select count(*) as co from `units` where `price` BETWEEN '$a4' and '$a5' and `citye` like '$a1' and `type1e` like '$a2' and `type2e` like '$a3' ")));
if(isset($_GET['start'])){


$start =$_GET['start'];
}else{$start=0;}
$limit =12;
$total =$co;
$num_page =ceil($total /$limit);



$data = mysql_query("select * from `units` where `price` BETWEEN '$a4' and '$a5' and `citye` like '$a1' and `type1e` like '$a2' and `type2e` like '$a3' limit $start ,$limit ") or die (mysql_error());

$num = mysql_num_rows($data);

if ($num






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