How to use PHP's built-in classes in namespaces

How do I use PHP's built-in classes in namespaces?
Hello, beginner PHP, encounter problems, ask for advice:
This is the code in a php file. Contains 2 namespaces, Test2 and Test4, Exception class throws an error in Test2
Errors are detected in the test4 space, but the runtime says that the exception class is not found in test2, so the exception class in the Red section
The previous add \ Indicates the root space, but in test4 the catch statement Exception $e error
How can I change the correct, thank you.
Namespace test2{
Class a{
Private $file _id;

Public Function File_open ($path _file_name) {
$this->[email protected] ($path _file_name, ' R ');
if ($this->file_id==false) {throw new \exception ("Path_file_name isnot EXIST");
Echo (NL2BR (sprintf ("File Open failed\n")); }
}
}
}

Namespace test4{
Use Test2;
try{
$obj _a = new Test2\a ();
$path _name = "/home/zhiys/php/111.php";
$obj _a->file_open ($path _name);
}catch (Exception $e) {...}

}

------Solution--------------------
catch (\exception $e)

