PHP で 2 つの郵便番号間の距離を計算するにはどうすればよいですか?

<code class="php">function calc_distance($point1, $point2)
{
    // Constants
    $radius       = 3958;      // Earth's radius (miles)
    $deg_per_rad  = 57.29578;  // Number of degrees/radian (for conversion)

    $distance = ($radius * pi() * sqrt(
        ($point1['lat'] - $point2['lat'])
        * ($point1['lat'] - $point2['lat'])
        + cos($point1['lat'] / $deg_per_rad)  // Convert to radians for cos()
        * cos($point2['lat'] / $deg_per_rad)  // Convert to radians for cos()
        * ($point1['long'] - $point2['long'])
        * ($point1['long'] - $point2['long'])
    ) / 180);

    return $distance;  // Returned using the units used for $radius.
}</code>