When you open phpinfo() see if the thread safety is enable or not enable also see the version of php and see MSVC-- what is the number in place of -- and see the architecture these all things help u to get the suitable php driver
here a url help u to get a php driver https://s3.amazonaws.com/drivers.mongodb.org/php/index.html
if your php version is Non-Thread-Safe (nts) you must use php extension with format example: extension=php_cl_dbg_5_2_nts.dll else if your php version is Thread-Safe (ts) you must use php extension with format example: extension=php_cl_dbg_5_2_ts.dll (notice bolded words)
So if get error like above. Firstly, check your PHP version is nts or ts, if is nts.
Then check in php.ini whether has any line like zend_extension_ts="C:\xammp\php\ext\php_dbg.dll-5.2.x" choose right version of php_dbg.dll-5.2.x from it homepage (google for it).
Change from zend_extension_ts to zend_extension_nts.
Hope this help.
Download php5.dll (http://windows.php.net/download/) and copy it to apache/bin folder. That solved it for me (Win 7 64 bit apache 32 bit)
EDIT: Start with the non-thread safe version.