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<?php

$db = mysql_connect('localhost', 'root', 'root') or die('Can\'t connect to database');
mysql_select_db('test') or die('Can\'t find database : test');
$result = mysql_query('select id, fid, name from tree');
while($arr = mysql_fetch_array($result)){
    $data[] = array(
        'id' => $arr['id'], 
        'fid' => $arr['fid'],
        'name' => $arr['name'], 
    );
}

// 将数据按照缩进简单排列 见图1
function data2arr($tree, $rootId = 0, $level = 0) {
    foreach($tree as $leaf) {
        if($leaf['fid'] == $rootId) {
            echo str_repeat('&nbsp;&nbsp;&nbsp;&nbsp;', $level) . $leaf['id'] . ' ' . $leaf['name'] . '<br/>';
            foreach($tree as $l) {
                if($l['fid'] == $leaf['id']) {
                    data2arr($tree, $leaf['id'], $level + 1);
                    break;
                }
            }
        }
    }
}

data2arr($data);
echo '<br/>-----------------------------------<br/>';

// 将数据按照所属关系封装 见图2
function arr2tree($tree, $rootId = 0) {
    $return = array();
    foreach($tree as $leaf) {
        if($leaf['fid'] == $rootId) {
            foreach($tree as $subleaf) {
                if($subleaf['fid'] == $leaf['id']) {
                    $leaf['children'] = arr2tree($tree, $leaf['id']);
                    break;
                }
            }
            $return[] = $leaf;
        }
    }
    return $return;
}

$tree = arr2tree($data);
print_r($tree);
echo '<br/>-----------------------------------<br/>';

// 将数据使用HTML再次展现 见图3
function tree2html($tree) {
    echo '<ul>';
    foreach($tree as $leaf) {
        echo '<li>' .$leaf['name'];
        if(! empty($leaf['children'])) tree2html($leaf['children']);
        echo '</li>';
    }
    echo '</ul>';
}

tree2html($tree);

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图 1：

?图 2：

??图 3：