$a=array(
        array('id'=>1,'location'=>8),
        array('id'=>2,'location'=>7),
        array('id'=>3,'location'=>8),
        array('id'=>4,'location'=>8),
        array('id'=>5,'location'=>8),
    );
    $b=array_flip(array_column($a,'location','id'));
        foreach($b as $key=>$val){
            $res = array();
            foreach($a as $k=>$v){
                if($v['location'] == $key){
                    unset($v['location']);
                    array_push($res,$v);
                }
            }
            $b[$key] = $res;
        }