Geo Search Without Distance
10 km in a straight line is: on the longitude is equal to ~6 minutes
The distance to longitude ratio is not a constant - it depends on the the user's latitude. You can use this function for the longitude difference:
function distanceToLng($distance,$latDeg) { $latRad = $latDeg / 180 * M_PI; $R = 6371; return atan2(1*sin($distance/$R)*cos($latRad), cos($distance/$R) - sin($latRad)*sin($latRad)) * (180 / M_PI);}The formula is taken from this question.
You can calculate latitude difference from a given distance using something like this:
$lngDeg = $distanceInKm/6371 * 180 / M_PI
As the other answer noted you really want to use the Haversine Formula which takes into account the users latitude.
Haversine Formula: http://en.wikipedia.org/wiki/Haversine_formula
This other answer has a ton of great resources to accomplish this in PHP or SQL: