function distance(float $lat1, float $lon1, float $lat2, float $lon2, string $unit): float
{
    $theta = $lon1 - $lon2;
    $dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
    $dist = acos($dist);
    $dist = rad2deg($dist);
    $miles = $dist * 60 * 1.1515;
    $unit = strtoupper($unit);

    switch ($unit) {
        case 'M':
            return $miles;
        case 'K':
            return $miles * 1.609344;
        case 'N':
            return $miles * 0.8684;
    }
}

// Rotterdam
$lat1 = 51.924419;
$lon1 = 4.47917;

// Amsterdam
$lat2 = 52.370216;
$lon2 = 4.477733;

Miles

distance($lat1, $lon1, $lat2, $lon2, 'M'); // 30.800174981173

Kilometers

distance($lat1, $lon1, $lat2, $lon2, 'K'); // 49.568076804901

Nautical Miles

distance($lat1, $lon1, $lat2, $lon2, 'N'); // 26.746871953651