Given an instance of a nested class, get_class() returns the ":: separated" name for it, as expected. However, passing that value right back to class_exists() always fails -- i.e., returns FALSE.

<?php

class Foo {
    class Bar {
    }
}

$x = new Foo::Bar;
if (class_exists(get_class($x))) {
    echo "x is looking good\n";
} else {
    echo "x is apparently not itself\n";
}

?>

In addition, get_declared_classes() will only report on the top-level classes, leaving out any that are nested. In this example, it would report "Foo", but not "Foo::Bar".