如何在php中使用'使用'，导致致命错误

I have learned that I need to use name space in php like I have used in my following code but I'm getting Fatal error: Trait 'SuperClosure\Serializer' not found I have used it like this:

use SuperClosure\Serializer;
public function set( $key, $value )
   {
    $key = strtolower( $key );      
    $serializer = new Serializer();
    $serialized = $serializer->serialize($value);



    $_SESSION["HA::STORE"][$key] = $serialized;
   }

where I am wrong ? please tell me the correct way to use it?

You need to use namespaces before declaring class because word use point to Trait mechanism (PHP: Traits). Example of namespaces and traits in class:

<?php namespace Foo\Bar;

/* I am telling to PHP compiler that class
 * Serializer has different path than class Baz.
 */
use SuperClosure\Serializer;
use SuperClosure\Shortcuts;

class Baz {
    /* Now I used Trait. That should include
     * methods defined in trait Shortcuts
     */
    use Shortcuts;
}

Also I recommend read more about namespaces and importing them.