关于计算周围多少里以内的楼盘

    /**
     * 根据经纬度计算距离 其中A($lat1,$lng1)、B($lat2,$lng2)
         * 注意弧度角度的计算
     * 单位：km
     */
    function _getDistance($lat1,$lng1,$lat2,$lng2)
    {
        //地球半径
        $R = 6378.137; //km
    
        //将角度转为狐度
        $radLat1 = deg2rad($lat1);
        $radLat2 = deg2rad($lat2);
        $radLng1 = deg2rad($lng1);
        $radLng2 = deg2rad($lng2);
    
        //结果
        $s = acos(cos($radLat1)*cos($radLat2)*cos($radLng1-$radLng2)+sin($radLat1)*sin($radLat2))*$R;
    
        //精度
        $s = round($s* 10000)/10000;
        return  round($s);
    }

/**
 *根据传入的中心点的经纬度和半径，计算出矩形区域
 * @param float $center_lat
 * @param float $center_lng
 * @param int   $radius unit:km
 */
function getAroundRectangle($center_lat, $center_lng, $radius)
{
    //先来求东西两侧的的范围边界 经度
    $earth_radius = 6378.137;    //km
    $dlng = rad2deg(2 * asin(sin($radius / (2 * $earth_radius)) / cos(deg2rad($center_lat)))); //角度
     
    //然后求南北两侧的范围边界 维度
    $dlat = rad2deg($radius/$earth_radius);
    
    $data = array(
        'lat_min' => $center_lat-$dlat,//维度最小
        'lat_max' => $center_lat+$dlat,//唯独 最大
        'lng_min' => $center_lng-$dlng,//经度最小
        'lng_max' => $center_lng+$dlng,//经度最大
    );
    return $data;
}

/**
 * Geohash generation class
 * http://blog.dixo.net/downloads/
 *
 * This file copyright (C) 2008 Paul Dixon (paul@elphin.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 3
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 */



/**
* Encode and decode geohashes
*
*/
class Geohash
{
    private $coding="0123456789bcdefghjkmnpqrstuvwxyz";
    private $codingMap=array();
    
    public function Geohash()
    {
        //build map from encoding char to 0 padded bitfield
        for($i=0; $i<32; $i++)
        {
            $this->codingMap[substr($this->coding,$i,1)]=str_pad(decbin($i), 5, "0", STR_PAD_LEFT);
        }
        
    }
    
    /**
    * Decode a geohash and return an array with decimal lat,long in it
    */
    public function decode($hash)
    {
        //decode hash into binary string
        $binary="";
        $hl=strlen($hash);
        for($i=0; $i<$hl; $i++)
        {
            $binary.=$this->codingMap[substr($hash,$i,1)];
        }
        
        //split the binary into lat and log binary strings
        $bl=strlen($binary);
        $blat="";
        $blong="";
        for ($i=0; $i<$bl; $i++)
        {
            if ($i%2)
                $blat=$blat.substr($binary,$i,1);
            else
                $blong=$blong.substr($binary,$i,1);
            
        }
        
        //now concert to decimal
        $lat=$this->binDecode($blat,-90,90);
        $long=$this->binDecode($blong,-180,180);
        
        //figure out how precise the bit count makes this calculation
        $latErr=$this->calcError(strlen($blat),-90,90);
        $longErr=$this->calcError(strlen($blong),-180,180);
                
        //how many decimal places should we use? There's a little art to
        //this to ensure I get the same roundings as geohash.org
        $latPlaces=max(1, -round(log10($latErr))) - 1;
        $longPlaces=max(1, -round(log10($longErr))) - 1;
        
        //round it
        $lat=round($lat, $latPlaces);
        $long=round($long, $longPlaces);
        
        return array($lat,$long);
    }

    
    /**
    * Encode a hash from given lat and long
    */
    public function encode($lat,$long)
    {
        //how many bits does latitude need?    
        $plat=$this->precision($lat);
        $latbits=1;
        $err=45;
        while($err>$plat)
        {
            $latbits++;
            $err/=2;
        }
        
        //how many bits does longitude need?
        $plong=$this->precision($long);
        $longbits=1;
        $err=90;
        while($err>$plong)
        {
            $longbits++;
            $err/=2;
        }
        
        //bit counts need to be equal
        $bits=max($latbits,$longbits);
        
        //as the hash create bits in groups of 5, lets not
        //waste any bits - lets bulk it up to a multiple of 5
        //and favour the longitude for any odd bits
        $longbits=$bits;
        $latbits=$bits;
        $addlong=1;
        while (($longbits+$latbits)%5 != 0)
        {
            $longbits+=$addlong;
            $latbits+=!$addlong;
            $addlong=!$addlong;
        }
        
        
        //encode each as binary string
        $blat=$this->binEncode($lat,-90,90, $latbits);
        $blong=$this->binEncode($long,-180,180,$longbits);
        
        //merge lat and long together
        $binary="";
        $uselong=1;
        while (strlen($blat)+strlen($blong))
        {
            if ($uselong)
            {
                $binary=$binary.substr($blong,0,1);
                $blong=substr($blong,1);
            }
            else
            {
                $binary=$binary.substr($blat,0,1);
                $blat=substr($blat,1);
            }
            $uselong=!$uselong;
        }
        
        //convert binary string to hash
        $hash="";
        for ($i=0; $i<strlen($binary); $i+=5)
        {
            $n=bindec(substr($binary,$i,5));
            $hash=$hash.$this->coding[$n];
        }
        
        
        return $hash;
    }
    
    /**
    * What's the maximum error for $bits bits covering a range $min to $max
    */
    private function calcError($bits,$min,$max)
    {
        $err=($max-$min)/2;
        while ($bits--)
            $err/=2;
        return $err;
    }
    
    /*
    * returns precision of number
    * precision of 42 is 0.5
    * precision of 42.4 is 0.05
    * precision of 42.41 is 0.005 etc
    */
    private function precision($number)
    {
        $precision=0;
        $pt=strpos($number,'.');
        if ($pt!==false)
        {
            $precision=-(strlen($number)-$pt-1);
        }
        
        return pow(10,$precision)/2;
    }
    
    
    /**
    * create binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example
    * removing the tail recursion is left an exercise for the reader
    */
    private function binEncode($number, $min, $max, $bitcount)
    {
        if ($bitcount==0)
            return "";
        
        #echo "$bitcount: $min $max<br>";
            
        //this is our mid point - we will produce a bit to say
        //whether $number is above or below this mid point
        $mid=($min+$max)/2;
        if ($number>$mid)
            return "1".$this->binEncode($number, $mid, $max,$bitcount-1);
        else
            return "0".$this->binEncode($number, $min, $mid,$bitcount-1);
    }
    

    /**
    * decodes binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example
    * removing the tail recursion is left an exercise for the reader
    */
    private function binDecode($binary, $min, $max)
    {
        $mid=($min+$max)/2;
        
        if (strlen($binary)==0)
            return $mid;
            
        $bit=substr($binary,0,1);
        $binary=substr($binary,1);
        
        if ($bit==1)
            return $this->binDecode($binary, $mid, $max);
        else
            return $this->binDecode($binary, $min, $mid);
    }
}

方案1：

　　根据中心点，和 上面的算法计算出几公里以内的最大/最小经纬度，然后搜索时用这个条件 （我们想要的为圆型的，需要过滤一次数据在），使用于数据量相对较小的

缺点：1.范围比较的索引利用率并不高，2.SQL语句极其不稳定(不同的当前位置会产生完全不同的SQL查询)，很难缓存。

方案2：

　　运用geohash， geohash是一种地址编码，它能把二维的经纬度编码成一维的字符串， 字符串匹配度越大，离的越近，适用于数据量较大的，

缺点：匹配程度并不能准确控制距离，只能找出比他大的范围，然后在用程序去判断

文章链接：

http://tech.idv2.com/2011/06/17/location-search/

http://tech.idv2.com/2011/07/05/geohash-intro/

http://www.wubiao.info/401