Monitor a PHP variable for change in value - php

Question

I am actually trying to monitor a PHP variable (may be as a separate thread but not possible in PHP) and fire a PHP function whenever the value of the variable changes.
eg: lets take a variable $_GLOBALS['foo']=1;
if at any point in the code, the value of $_GLOBALS['foo'] changes to something else, i want to fire a PHP function immediately.
The variable can be anywhere inside a loop or in a function,etc.
Why i want this: I have a variable which stores the last error occured as a text. If the value of the variable changes from "" to something else, i want to trigger an error. My LOGIC may seem a bit strange but this is what i would like to do.
Thanx in advance.
Edit: I tried: How to use monitors in PHP? and How can one use multi threading in PHP applications but does not seem to solve the problem.
The Code (Thought this could solve some of your doubts on my question):
public function addtag($tagarray,$uid,$tagtype="1")
{
$dbobj=new dboperations();
$uiobj=new uifriend();
$tagid=$uiobj->randomstring(30,DB_SOCIAL,SOCIAL_TAG,tag_tagid);
$c=0;
foreach($tagarray as $tags)
{
$c++;
$tagname=$tags["tagname"];
$taguid=$tags["tagid"];
$dbobj->dbinsert("INSERT INTO ".SOCIAL_TAG." (".tag_tagid.",".tag_fuid.",".tag_tuid.",".tag_tagname.",".tag_tagtype.") VALUES
('$tagid','$uid','$taguid','$tagname','$tagtype')",DB_SOCIAL);
}
if($c==0)
{
$lasterror="No tags were added";return "";
}
else
{
return $tagid;
}
}
Here, if i call a error handling function instead of monitoring the variable, it wont be advisable in my case since the error handling function may do any operation like give alert and redirect to a page or any similar operation.
I asked this question cause, i thought what if the script does not reach the line
return "";
It would affect the project's workflow. Thats what i am worried about.
And the variable i was talking about is $lasterror and i have many functions like this where $lasterror is used.

Answer 1

I saw this, so I built this:
https://github.com/leedavis81/vent
Should solve your problem.

Answer 2

There is no built-in way to do this in PHP, and there's no easy way to add it. It doesn't really feel right for the way the language works anyway.
Instead of setting a variable, you could build a custom function that handles the error - or use PHP's built-in error handling functionality using a custom error handler.
Another error handling method which comes close to what you want to do (I think) is exceptions.