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PHP Recursion problem - value returns null... variable scope

Posted: Thu Jun 29, 2006 5:12 pm
by thedust2010
In my 3 years of PHP application development I don't think I've ever been this perplexed. It is rather difficult to explain, but hopefully it makes sense. For brevity I'm not giving details about the database table structures. Really it is irrelevant for what my problem is, which appears to be some sort of variable scope issue. The problem appears to be something really elementary...

I have a database-driven system of files and folders with permission to view these contents specified on a per-user basis. Occasionally this means assigning a folder a new parent folder if the user is blocked from seeing it. I've provided a visual aid that makes this much easier to comprehend:

Image

So here in this example we would be looking to assign "TEST2" as the parent of "INSIDE 4". To do this, I have a recursive function that finds the new parent ID that should be assigned. It looks like this:

Code: Select all

function findParentID($userID, $folderID) {
	global $DB;
	
	$SQL = "SELECT folderID FROM navAccessFolders WHERE folderID=".$folderID." AND userID=".$userID;
	$Row = $DB->GetRow($SQL);
	if ($Row) {
		echo "value to return: ".$folderID."<br>"; // for troubleshooting
		return $folderID;
	} else {
		$SQL = "SELECT parentID FROM folders WHERE folderID=".$folderID;
		$Row = $DB->GetRow($SQL);
		findParentID($userID, $Row["parentID"]);
	}
}
Now let's say the folder ID for "INSIDE 4" is 138. To get the parentID I would use:

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$parentID = findParentID(1, 138); // using "1" as a sample user ID
echo "value returned: ".$parentID."<br>"; // for troubleshooting
This functionality WORKS if the findParentID function never calls upon itself again. Then the value returns null. But here is the part I don't think anyone can explain to me. The actual output from the code used above is:

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value to return: 135
value returned:
I am echoing the value just before the function returns it and it IS in fact the correct value. However, it is not returning the value that I am seeing in the output. It returns null. How can this be? There is not one line of code between the echo and the return. And again, this ONLY is happening when the function calls upon itself again. The function works but the value returns null anyways.

If someone can figure this one out, you will have made my list of all-time top coders (and maybe we can hire you for some work sometime??). Any advice or help is much appreciated. I'm so close...

Posted: Thu Jun 29, 2006 5:33 pm
by Chris Corbyn
Place the keyword "return" infront of your recursive call.

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function findParentID($userID, $folderID) {
        global $DB;
       
        $SQL = "SELECT folderID FROM navAccessFolders WHERE folderID=".$folderID." AND userID=".$userID;
        $Row = $DB->GetRow($SQL);
        if ($Row) {
                echo "value to return: ".$folderID."<br>"; // for troubleshooting
                return $folderID;
        } else {
                $SQL = "SELECT parentID FROM folders WHERE folderID=".$folderID;
                $Row = $DB->GetRow($SQL);
                return findParentID($userID, $Row["parentID"]);
        }
}