In order to check class you must specify it with namespace, full path:
namespace Foo;
class Bar
{
}
and
var_dump(class_exists('Bar'), class_exists('FooBar')); //false, true
-i.e. you must specify full path to class. You defined it in your namespace and not in global context.
However, if you do import the class within the namespace like you do in your sample, you can reference it via imported name and without namespace, but that does not allow you to do that within dynamic constructions and in particular, in-line strings that forms class name. For example, all following will fail:
namespace Foo;
class Bar {
public static function baz() {}
}
use FooBar;
var_dump(class_exists('Bar')); //false
var_dump(method_exists('Bar', 'baz')); //false
$ref = "Bar";
$obj = new $ref(); //fatal
and so on. The issue lies within the mechanics of working for imported aliases. So when working with such constructions, you have to specify full path:
var_dump(class_exists('FooBar')); //true
var_dump(method_exists('FooBar', 'baz')); //true
$ref = 'FooBar';
$obj = new $ref(); //ok
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