php infinite loop - php

Question

This function gives me an infinite loop
function getCats($parent,$level){
// retrieve all children of $parent
$result = "";
$query = "SELECT title,parent_id from t_cats where parent_id = '$parent'";
if($rs = C_DB::fetchRecordset($query)){
while($row = C_DB::fetchRow($rs)){
$result .= str_repeat($parent,$level).$row['title']."\n";
getCats($row['parent_id'],$level+1);
}
}
return $result;
}
here is my db table
CREATE TABLE `db`.`t_cats` (
`ID` int(10) unsigned NOT NULL auto_increment,
`datasource_id` int(10) unsigned default '0',
`version` char(10) character set latin1 default 'edit',
`status` char(10) character set latin1 default 'new',
`modified_date` datetime default NULL,
`modified_by` int(10) unsigned default '0',
`title` char(255) character set latin1 default NULL,
`parent_id` int(11) default NULL,
PRIMARY KEY (`ID`),
KEY `idx_datasource_id` (`datasource_id`)
) ENGINE=MyISAM AUTO_INCREMENT=50 DEFAULT CHARSET=utf8;
I just want to be able to get my list of categories recursive.
But what am i doing wrong?
EDIT:
function getCats($parent,$level){
// retrieve all children of $parent
$result ="";
$query = "SELECT title,parent_id from t_cats where parent_id = '$parent'";
if($rs = C_DB::fetchRecordset($query)){
while($row = C_DB::fetchRow($rs)){
$result.= str_repeat($parent,$level).$row['title']."\n";
getCats($row['id'],$level + 1 );
}
}
return $result;
}

Answer 1

This line looks wrong:
getCats($row['parent_id'],$level+1);
You should be calling it with the current child ID - at the moment you're calling it with the same ID over and over. Something like this (you need to select the id from your table):
getCats($row['id'], $level + 1);
Edit: you need to update your query to select id:
$query = "SELECT id, title, parent_id from t_cats where parent_id = '$parent' AND id != parent_id";
I've also added a bit to stop it getting into a loop if an item is its own parent.

Answer 2

I found this SitePoint article on "Storing Hierarchical Data in a Database" very helpful. It's all PHP examples, and it will improve the performance of what you're trying to do dramatically.

Answer 3

Maybe one of the items in the db has itself as parent?

Answer 4

I don't know C_DB, but I'd bet that the $rs returned by fetchrecordset is a reference, which means that every invocation of getCats is using the same $rs. Exactly what it will do then is unpredictable, depending on how fetchRow is implemented.
If you want to do this (and recursive closures are a pain in SQL, I know), you should open a new connection inside getCats. and be using a separate connection for each access.

Answer 5

correct answer provided by greg ...
2 side notes:
in the case of infinite loops, track recursion depth (you can conveniently use $level here) or overall invocation count (if you are lazy, since this is a oneliner accessing a global counter), and terminate recursion with an exception, when it reaches a maximum value (in general, 10 is already enough to see the problem, but of course that may vary) ... and then get some debug output ... for example $query or something like "calling getCats($parent,$level)" ... would've shown you the problem in no time in this case ... :)
you should minimize the amount of queries ... traversing a tree like that is quite inefficient ... especially, if the database is on another machine ...
greetz
back2dos

Answer 6

Erm shouldnt it be:
$query = "SELECT title,parent_id from t_cats where id = '$parent'";
And not:
$query = "SELECT title,parent_id from t_cats where parent_id = '$parent'";