PHP recursive algorithm PHP recursive function infinite classification

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/**
* Function: Query all categories to generate Tree menu
* @param int $pid parent ID, query starts from the top level by default
*/
function toTree($pid=0){
 //Query all top categories
 //Not much to say about the database connection here
 $model=M('Category'); Instantiate the model
 //Query map conditions
 $map=array(
 'pid'=>$pid,
 );
 //Query
 $data=$model- >where($map)->select();
 //Define a new array to store the generated html tree menu
 $html_array=array();
 //Loop classification
 $html='';
 $padding=0; //Indentation
 foreach($data as $k=>$v){
 //Indentation is implemented based on the value of the path field
 $path=$v['path'];
 if( $path&&strpos($v['path'],'-')){
 $path_array=@explode('-',$v['path']);
 $count=count($path_array)-1;
 $padding=$count*20; //The next level classification is indented by 20 pixels
 }
 //Current classification data html
 $data_array[]='
'.$v['name'].'
';< ;/p>
 //Recursively start to search for lower-level categories
 $data_array[]=toTree($v['id']);//Recurse using the ID of the current category as the parent ID
 }
 / /Foreach loop ends and merges classified html arrays
 $html.=implode('',$data_array());
 $html.='';
 //HTML tree menu construction is completed, it’s that simple, so easy!
 return $html;
}