递归无限级分类有几种形式，我这里仅仅举例比較经常使用的三种：

第一种：返回有排序的数组：

<?php

$data = array(

    1 => array(

        'id' => 1,

        'pid' => 0,

        'user_name' => 'one',

    ),

    2 => array(

        'id' => 2,

        'pid' => 1,

        'user_name' => 'two',

    ),

    3 => array(

        'id' => 3,

        'pid' => 1,

        'user_name' => 'two',

    ),

    4 => array(

        'id' => 4,

        'pid' => 2,

        'user_name' => 'three',

    ),

    5 => array(

        'id' => 5,

        'pid' => 2,

        'user_name' => 'three',

    ),

    6 => array(

        'id' => 6,

        'pid' => 5,

        'user_name' => 'four',

    ),

);

function genCate( $data, $pid = 0, $level = 0 ) {

    static $result = array();

    $result = ($level == 0) ? array() : $result;

    $prefix = ($level == 0) ? '' : str_repeat( '-', $level );

    foreach ( $data as $key => $row ) {

        if ( $row['pid'] == $pid ) {

            $row['user_name'] = $prefix . $row['user_name'];

            $result[] = $row['user_name'];

            genCate( $data, $row['id'], $level + 1 );

        }

    }

    return $result;

}

echo '<pre>';

$result = genCate( $data );

print_r( $result );

exit;

?>

另外一种：返回html字符串形式：

function getTreeHtml( $data, $pId ) {

    $html = '';

    foreach ( $data as $k => $v ) {

        if ( $v['pid'] == $pId ) {

            $html .= "<li>" . $v['user_name'];

            $html .= getTree( $data, $v['id'] );

            $html = $html . "</li>";

        }

    }

    return $html ? '<ul>' . $html . '</ul>' : $html;

}

第三种：返回多维数组形式：

function getTreeArray( $data, $pId = 0 ) {

    $tree = array();

    foreach ( $data as $key => $value ) {

        if ( $value['pid'] == $pId ) {

            $value['childrens'] = getTreeArray( $data, $value['id'] );

            $tree[] = $value;

        }

    }

    return $tree;

}