PHP development classification technology uses recursion to achieve infinite classification (1)

First, we need to create a simple database test

<?php
$link = mysqli_connect('localhost','username','password','test');
mysqli_set_charset($link, "utf8");
if (!$link) {
  die("连接失败:".mysqli_connect_error());
}
?>

Create a new table class and set 3 fields

Sort id int type.

Category name title varchar type.

Classification pid int type.

A table similar to the following:

Idea:

Define a custom function get_str, set the parent class pid = 0, use SQL The statement queries out its subclasses, Place the queried subclasses into $result

Use a while loop to get out the subclasses, and create an output pattern by constructing a string , call the custom function get_str, pass the id of the subclass into the custom function,

and then continue to query the next level.

<?php
function get_str($id = 0) {
  global $str;
  global $link;    //global 关键词用于访问函数内的全局变量。
  $sql = "select id,title from class where pid= $id";
  $result = mysqli_query($link,$sql);//查询pid的子类的分类
  if($result){//如果有子类
    $str .= '<ul>';
    while ($row = mysqli_fetch_array($result)) { //循环记录集
      $str .= "<li>" . $row['id'] . "--" . $row['title'] . "</li>"; //构建字符串
      get_str($row['id']); //调用get_str()，将记录集中的id参数传入函数中，继续查询下级
    }
    $str .= '</ul>';
  }
  return $str;
}
echo get_str(0);
?>

The output is similar to: