php+mysql Infinite class instance without recursive implementation of non recursive

Recursion 1 is generally the first and easiest to think of to achieve infinite classification, but recursion 1 is generally considered as a resource-consuming method, so many systems do not consider using recursion
In this paper, the database is designed and implemented with an sql statement
The database fields are roughly as follows:

id  Numbering 

fid  Parent classification number 

class_name  Classification name 

path  Classification path to  id  Is a node with a similar composition  ,1,2,3,4,  Such a string 

The following data can be assumed:

id fid class_name path

1  0        Classification 1 ,       1,

2  0        Classification 2 ,       2,

3  1        Classification 1-1 ,    1,3,

4  1        Classification 1-2 ,    1,4,

5  2        Classification 2-1 ,    2,5,

6  4        Classification 1-2-1 , 1,4,6,

Unlimited classification operation code:

<?php   
$sql= " SELECT * FROM tree order by path " ;  
$result=$nbs->Query($sql);  
while($rows=$nbs->fetch_array($result)){  
  if(substr_count($rows['path'],',')>2){  
    for($i=0;$i<(substr_count($rows['path'],',')-2);$i++)  
      echo  '   ' ;  
  }  
  echo $rows['class_name'].'<br>';  
}  
?>  

$conn = mysql_connect ( 'localhost', 'root', 'root' );
mysql_select_db ( 'wanggou123', $conn );
mysql_query ( 'set names UTF8' );
$sql = "select id,concat(catpath,'-',id) as abspath,name from category order by abspath";
 
$query = mysql_query ( $sql );
while ( $row=mysql_fetch_array($query)) {
  /**
  *  No. 1 1 Species display method 
*/
/*$space = str_repeat ( '&nbsp;&nbsp;&nbsp;&nbsp;', count ( explode ( '-', $row ['abspath'] ) ) - 1 );
echo $space . $row ['name'] . '
';*/
/**
  No. 1 2 Species display method 
*/
$space = str_repeat ( ' - ', count ( explode ( '-', $row ['abspath'] ) ) - 1 );
$option .= '' . $space . $row ['name'] . '<Br>';
}
echo $option;
exit();
echo '<select name="opt">' . $option . '</select>';

Among them, $nbs is a database operation class, which is simple and clear!