运行脚本时的空白页面

Question

I do these equations

$longitude = (float) $lat_from_db;
$latitude = (float) $lon_from_db;
$radius = $radius_from_post;

$lng_min = $longitude - $radius / abs(cos(deg2rad($latitude)) * 69);
$lng_max = $longitude + $radius / abs(cos(deg2rad($latitude)) * 69);
$lat_min = $latitude - ($radius / 69);
$lat_max = $latitude + ($radius / 69);

They spit out the proper coordinance and work.... I then put them into MySQL like this:

$sql = "SELECT * FROM zip_code WHERE (lon BETWEEN '$lng_min'AND '$lng_max') AND (lat BETWEEN '$lat_min' and '$lat_max')";
$zip_code_array = mysql_query($sql);
while($zip_code_cells = mysql_fetch_array($zip_code_array))
{
    $zip_codes_in_distance = $zip_code_cells['zip_code'];
}

echo $zip_codes_in_distance;

I run it, it echos a blank page.