Namespaces in PHP (with Examples)

The problem namespaces solve

In a big project, you might have your own User class, and a library you installed might also have a User class. PHP cannot have two classes with the exact same name — it causes a "cannot redeclare" error. Namespaces fix this by giving code a group name, like a surname, so the two User classes can coexist.

Defining a namespace

Declare it as the first statement in the file.

<?php
namespace App\Models;

class User {
    public function greet() {
        return "Hello from App\\Models\\User";
    }
}
?>

Now this User belongs to App\Models. Another User in a different namespace will not clash with it.

Using it with use

Import the class with use, then call it by its short name.

<?php
require "User.php";
use App\Models\User;

$u = new User();
echo $u->greet();
?>

Without use, you would have to write the full path every time: new \App\Models\User(). The use line keeps your code short and clean.

Aliasing with as

If two classes share a name, rename one locally with as.

<?php
use App\Models\User as AppUser;
use Vendor\Auth\User as AuthUser;

$a = new AppUser();
$b = new AuthUser();
?>

Now both "User" classes are usable in the same file under different local names — no clash, no confusion.

Sub-namespaces

Namespaces can be nested with backslashes to mirror your folder structure.

<?php
namespace App\Controllers\Admin;
// full name: App\Controllers\Admin\DashboardController
class DashboardController { }
?>

By convention App\Controllers\Admin matches the folder app/Controllers/Admin/. This tidy matching is what lets autoloaders find classes automatically.

Common mistakes

• namespace not first. The namespace line must be the first statement after <?php.
• Forgetting use. Then you must write the full path or PHP cannot find the class.
• Mixing / and \. Namespaces use backslashes \, not forward slashes.
• Name clash after import. Two same-named imports need as to rename one.

Summary

• Namespaces group code so classes with the same name do not clash.
• Declare with namespace App\Models; as the first statement.
• use imports a class so you can use its short name.
• as renames an import to avoid clashes.
• Namespaces usually mirror folders and power autoloading.

Next, load namespaced classes automatically with PHP autoloading, or structure code with the MVC pattern.