在实际工作中，一个递归方法被重复调用的话，如果使用的static方式，由于上一次的数据并不会消失，第二次调用会造成数据的混乱。所以出现这种情况，需要换一种方式来解决。这里用的&引用传递来代替上一种方法。
下面是网上的一个例子，贴出来大家看下：
$items = array(
1 => array('id' => 1, 'pid' => 0, 'name' => '江西省'),
2 => array('id' => 2, 'pid' => 0, 'name' => '黑龙江省'),
3 => array('id' => 3, 'pid' => 1, 'name' => '南昌市'),
4 => array('id' => 4, 'pid' => 2, 'name' => '哈尔滨市'),
5 => array('id' => 5, 'pid' => 2, 'name' => '鸡西市'),
6 => array('id' => 6, 'pid' => 4, 'name' => '香坊区'),
7 => array('id' => 7, 'pid' => 4, 'name' => '南岗区'),
8 => array('id' => 8, 'pid' => 6, 'name' => '和兴路'),
9 => array('id' => 9, 'pid' => 7, 'name' => '西大直街'),
10 => array('id' => 10, 'pid' => 8, 'name' => '东北林业大学'),
11 => array('id' => 11, 'pid' => 9, 'name' => '哈尔滨工业大学'),
12 => array('id' => 12, 'pid' => 8, 'name' => '哈尔滨师范大学'),
13 => array('id' => 13, 'pid' => 1, 'name' => '赣州市'),
14 => array('id' => 14, 'pid' => 13, 'name' => '赣县'),
15 => array('id' => 15, 'pid' => 13, 'name' => '于都县'),
16 => array('id' => 16, 'pid' => 14, 'name' => '茅店镇'),
17 => array('id' => 17, 'pid' => 14, 'name' => '大田乡'),
18 => array('id' => 18, 'pid' => 16, 'name' => '义源村'),
19 => array('id' => 19, 'pid' => 16, 'name' => '上坝村'),
);function genTree($items) {
$tree = array(); //格式化好的树
foreach ($items as $item)
if (isset($items[$item['pid']]))
$items[$item['pid']]['son'][] = &$items[$item['id']];
else
$tree[] = &$items[$item['id']];
return $tree;
}打印结果，太长了，只贴一部分。
Array([0] => Array (
[id] => 1
[pid] => 0
[name] => 江西省
[son] => Array
(
[0] => Array
(
[id] => 3
[pid] => 1
[name] => 南昌市
)
[1] => Array
(
[id] => 13
[pid] => 1
[name] => 赣州市
[son] => Array
(
[0] => Array
(
[id] => 14
[pid] => 13
[name] => 赣县
[son] => Array
(
[0] => Array
(
[id] => 16
[pid] => 14
[name] => 茅店镇
[son] => Array
(
[0] => Array
(
[id] => 18
[pid] => 16
[name] => 义源村
)
[1] => Array
(
[id] => 19
[pid] => 16
[name] => 上坝村
)
)
)
[1] => Array
(
[id] => 17
[pid] => 14
[name] => 大田乡
)
)
)
[1] => Array
(
[id] => 15
[pid] => 13
[name] => 于都县
)
)
)
)
)
[1] => Array (
)
)*/第一次看到这个方法时我没看懂，今天回答贴子的问题时，也尝试了用&，才明白。
&符号：$a=&$b , $b变量的内存地址与$a变量的内存地址,指向了同一个地方.
这样，已经生成的数组，会随着后面值的变化不断变化，比如$arr['son'] = &$array, 虽然此次循环已经完毕，但是$arr['son'] 指向了$array同一个地方，再后面的循环中，会随着$array值的变化，随时随地的变化。由此顶级分类不断的壮大，最终组装成想要的结构。形成了数据的同步。
为了更好的说明&的作用。下面是我刚作的一个例子。
$data[] = array('id'=>"1601");
$data[] = array('id'=>"160101");
$data[] = array('id'=>"160102");
$data[] = array('id'=>"16010201");
$data[] = array('id'=>"1601020101");
$data[] = array('id'=>"1601020102");
$data[] = array('id'=>"160103");
$data[] = array('id'=>"16010301");
foreach ($data as $k => $v) {
$arr[ $v['id'] ] = $v;
}
foreach ($arr as $k => &$v) {
for($i=1;$i<100;$i++)
{
$oo = $i<10?'0'.$i:$i;
$sub_k = $k.$oo;
if( isset( $arr[$sub_k] ) )
{
$v['child'][$sub_k] = &$arr[$sub_k];//这里使用了&，数据实时同步。
$temp[] = $sub_k;
}
}
}
//如测试&注释掉下面的
/*for($i=0;$i<count($temp);$i++){unset($arr[$temp[$i]]);
}*/
如果不用&，会啥样呢？
Array
(
[1601] => Array
(
[id] => 1601
[child] => Array
(
[160101] => Array
(
[id] => 160101
)
[160102] => Array
(
[id] => 160102
)
[160103] => Array
(
[id] => 160103
)
)
)
[160101] => Array
(
[id] => 160101
)
[160102] => Array
(
[id] => 160102
[child] => Array
(
[16010201] => Array
(
[id] => 16010201
)
)
)
[16010201] => Array
(
[id] => 16010201
[child] => Array
(
[1601020101] => Array
(
[id] => 1601020101
)
[1601020102] => Array
(
[id] => 1601020102
)
)
)
[1601020101] => Array
(
[id] => 1601020101
)
[1601020102] => Array
(
[id] => 1601020102
)
[160103] => Array
(
[id] => 160103
[child] => Array
(
[16010301] => Array
(
[id] => 16010301
)
)
)
[16010301] => Array
(
[id] => 16010301
)
)
加上&：
Array
(
[1601] => Array
(
[id] => 1601
[child] => Array
(
[160101] => Array
(
[id] => 160101
)
[160102] => Array
(
[id] => 160102
[child] => Array
(
[16010201] => Array
(
[id] => 16010201
[child] => Array
(
[1601020101] => Array
(
[id] => 1601020101
)
[1601020102] => Array
(
[id] => 1601020102
)
)
)
)
)
[160103] => Array
(
[id] => 160103
[child] => Array
(
[16010301] => Array
(
[id] => 16010301
)
)
)
)
)
[160101] => Array
(
[id] => 160101
)
[160102] => Array
(
[id] => 160102
[child] => Array
(
[16010201] => Array
(
[id] => 16010201
[child] => Array
(
[1601020101] => Array
(
[id] => 1601020101
)
[1601020102] => Array
(
[id] => 1601020102
)
)
)
)
)
[16010201] => Array
(
[id] => 16010201
[child] => Array
(
[1601020101] => Array
(
[id] => 1601020101
)
[1601020102] => Array
(
[id] => 1601020102
)
)
)
[1601020101] => Array
(
[id] => 1601020101
)
[1601020102] => Array
(
[id] => 1601020102
)
[160103] => Array
(
[id] => 160103
[child] => Array
(
[16010301] => Array
(
[id] => 16010301
)
)
)
[16010301] => Array
(
[id] => 16010301
)
)
发现，加上&后，
结果：
这样就通过引用数据同步的原理，同样实现了递归的效果，而且引用的效率要比递归高。
有兴趣的朋友可以，拷贝下来，去掉&，看看是什么结果。加上&又是什么结果。
end;