How to use “root” namespace of php?

What can I do to make this work ?

Use a leading backslash to indicate the global namespace:

namespace abc;

class AbcException extends Exception {
// blah blah
}

Useful documents are appreciated.

There's an entire page devoted to this in the PHP manual!

To clear up any confusion regarding different syntax use, namespaces support only two syntaxes, either bracketed or simple-combination both will work. I suggest if you use one over the other, be consistent.

<?php
namespace mystuffnested {  // <- bracketed syntax
 class foo {}
}
?>

It creates a class foo inside of the nested namespace with bracketed syntax ({}), it is equivalent to

<?php
namespace mystuff {  // bracketed syntax but with a nested look
  namespace nested {
     class foo {}
  }
}
?>

You can also use nested namespaces with simple-combination syntax (;)

<?php
namespace mine;
use ultralongnsname;  // <- simple-combination syntax

$a = nameCONSTANT;
namefunc();
?>

PHP: FAQ: things you need to know about namespaces

The use operator is for giving aliases to names of classes, interfaces or other namespaces. Most use statements refer to a namespace or class that you'd like to shorten:

use MyFullNamespace;

is equivalent to:

use MyFullNamespace as Namespace;
// NamespaceFoo is now shorthand for MyFullNamespaceFoo

If the use operator is used with a class or interface name, it has the following uses:

// after this, "new DifferentName();" would instantiate a MyFullClassname
use MyFullClassname as DifferentName;

// global class - making "new ArrayObject()" and "new ArrayObject()" equivalent
use ArrayObject;

The use operator is not to be confused with autoloading. A class is autoloaded (negating the need for include) by registering an autoloader (e.g. with spl_autoload_register). You might want to read PSR-4 to see a suitable autoloader implementation.