Included files will default to the global namespace.
<?php
//test.php
namespace test {
  include 'test1.inc';
  echo '-',__NAMESPACE__,'-<br />';
}
?>

<?php
//test1.inc
  echo '-',__NAMESPACE__,'-<br />';
?>

Results of test.php:

--
-test-

In namespaced context the Exception class needs to be prefixed with global prefix operator.

<?php

namespace hey\ho\lets\go;

class MyClass
{
    public function failToCatch()
    {
        try {
            $thing = somethingThrowingAnException();
        } catch (Exception $ex) {
              // Not catched
        }
    }

    public function succeedToCatch()
    {
        try {
            $thing = somethingThrowingAnException();
        } catch (\Exception $ex) {
              // This is now catched
        }
    }

}

To define an associative array (hash) so that it is part of the namespace, instead of going into the global namespace, just declare it as const, istead of as a variable.

This is handy for lookup tables, config settings, etc.

// Example.  Instead of writing:

$my_datatypes = [
    "sterility" =>  [
      "xlsx" => [
        "Sample Type",
        "Run Pass/Fail",
        "Result"
       ],
      "db" => [
        "SampleType",
        "RunPassFail",
        "Result"
       ]
    ]
];

// ...declare the lookup table like this:

const MY_DATATYPES = [
    "sterility" =>  [
      "xlsx" => [
        "Sample Type",
        "Run Pass/Fail",
        "Result"
       ],
      "db" => [
        "SampleType",
        "RunPassFail",
        "Result"
       ]
    ]
];

// ...and it will be declared within the current namespace.

Note that variables aren't part of the namespace so they are always global (or scoped to function, etc.) and can't be accessed the same way as other namespace stuff.

So no:

    namespace Foo;
    $var = "hello";
    echo \Foo\$var;