php如何实现分类

CREATE TABLE `category` (
`id` smallint(5) unsigned NOT NULL auto_increment,
`fid` smallint(5) unsigned NOT NULL default '0',
`value` varchar(50) NOT NULL default '',
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

INSERT INTO `category` ( `fid`, `value`) VALUES
(1,'a'),
(1,'b'),
(2,'c'),
(2,'d'),
(4,'e')

  
    $conn = mysqli_connect("localhost", "root", "1234", 'tp51');
    if ($conn->connect_error) {
        die("连接失败: " . $conn->connect_error);
    }
    mysqli_set_charset($conn, "utf8");
    $sql = "SELECT * FROM category";
    $res = $conn->query($sql);
    if ($res->num_rows > 0) {
        while ($row = $res->fetch_assoc()) {
            $arr[] = array($row["id"],$row["fid"],$row["value"]);
        }
    } else {
        echo "0 结果";
    }
    mysqli_close($conn);
    getCate(1);
    function getCate($fid = 0)
    {
        global $arr;
        for ($i = 0; $i < count($arr); $i++) {
            if ($arr[$i][1] == $fid) {
                echo $arr[$i][2] . "<br>";
                getCate($arr[$i][0]); //递归
            }
        }
    }