PHP recursive implementation of infinite classification PHP format array

//I use mysql PDO for the database, but the whole idea is the same
$conn=mysql_connect('localhost','root','123');
 if(mysql_errno()){
 printf('Connection failed'.mysql_error());
}
mysql_select_db('edeng');
mysql_set_charset('utf8');
/*
 *Recursive function
 *@param id To query all subclasses of fid=$id, the default value of $id is set to 0 because I set the fid of the top-level category to 0 in the database
 */
function get_array($id=0){
 $sql="select id,fid,cname from e_cat where fid= $id";
 $result=mysql_query($sql);
 $arr=array();
 if($result && mysql_affected_rows()){
 while($rows=mysql_fetch_assoc($result)){
 $rows['child']=get_array($rows['id']);
 $arr[] = $rows;
 }
 return $arr;
 }
}
echo '';
$result = get_array();
print_r($result);
 < ;/P>
The function first queries all classes with fid 0
 Through the while loop one by one, the back investigation is performed to find the subclass whose fid is the id of the current class