So you or your team has built anywhere between 5 and 500 projects in PHP 4, 5.1 and 5.2 over the past 5 years. And now PHP 5.3 is there, offering a lot of very interesting features, including namespace support, late static binding (finally !), closures, nested exceptions and a bunch more (see the new feature list). So naturally, you'd like to upgrade. But doing so might break some old code.
He suggests a few different options - just run your unit tests and hope for the best, test the application's code directly or, his preference, run compatibility tests with the help of PHP_CodeSniffer and this new sniff he created. The sniff finds things like deprecated functions hanging around from pre-5.3 times as a part of a subset that the code sniffer can easily find.