While it is an ongoing project, the PHP library for AdWords does not yet fully make use of proper namespacing througout. Until that is complete, it may be advisable to use a "psedudo namespaced" version that simply prepends a bit of data to class names. Note that if you do it this way, you will want to use this format for creating objects. It's also a better practice anyway.
// Old
$tracker = new AdWordsConversionTracker();