什么是\用于PHP命名空间或类元素

Question

I been working on PHP framework for many years that I maintain for our internel closed source project.

While I updating this and keeping to coding standards with PHP 7 I notice that alot of developers use \ in there classes and namespaces that are called within the code.

for example

class ClassName extends ParentClass implements
    \ArrayAccess,
    \Countable,
    \Serializable
{
    // constants, properties, methods
}

or for example calling the class within functions of the classes

public function featured()
{
    // Check for request forgeries
    \Session::checkToken() or jexit(\Text::_('JINVALID_TOKEN'));

    $user   = \Factory::getUser();
    $ids    = $this->input->get('cid', [], 'array');
    $values = ['featured' => 1, 'unfeatured' => 0];
    $task   = $this->getTask();
    $value  = \ArrayHelper::getValue($values, $task, 0, 'int');
}

why is \ used and what is it purpose since it can work without \ as well.

as I cant see any information online what the use of the \ is for in classes and namespaces.

Answer 1

This is explained here: http://php.net/manual/en/language.namespaces.fallback.php

When executing a file, it has it's own namespace. Look at this code from the above page:

<?php
namespace A\B\C;
class Exception extends \Exception {}

$a = new Exception('hi'); // $a is an object of class A\B\C\Exception
$b = new \Exception('hi'); // $b is an object of class Exception

$c = new ArrayObject; // fatal error, class A\B\C\ArrayObject not found
?>

$a = new Exception('hi'); uses the Exception class in the current namespace i.e. A\B\C

$b = new \Exception('hi'); uses the global class Exception