How does thinkphp realize the circular retrieval of secondary menu?

ThinkPHP realizes the secondary loop reading as follows:

$ Category = D(' Category ')-& gt； Where ('category _ PID = 0')->; findAll()；

//dump($ Category)；

If (! Empty($Category)){ // Judge whether the first level is empty.

foreach($ Category as $ key = & gt； $value){ // loop reading

$ PID = $ value[' category _ id ']； //field assignment

$ Category[$ key][' child ']= D(' Category ')-& gt； Where ("category _ PID = $ PID")->; select()；

//echo D(' Category ')-& gt； getLastSql()； //Writing method of printing sql statement

}

//dump($ Category)；

$ this-& gt； assign('Category '，$ Category)； //Mapping value