Declaring and Using Namespaces

A namespace prefixes every class, interface, trait, enum, function and constant declared in it, so your Money and a library's Money can coexist. namespace must be the file's first statement (only declare() may precede it), and names run vendor first.

Declaring a namespace and how names resolve inside it (app/names.php)PHP

<?php
namespace Acme\ShopMoney;                   // the first statement (only declare() may precede)
const DEFAULT_LOCALE = 'en_US';             // defines \Acme\ShopMoney\DEFAULT_LOCALE
function cents(string $amount): int { return (int) round((float) $amount * 100); }
final class Money { public function __construct(public int $minor) {} }
echo __NAMESPACE__, ' | ', Money::class, ' | ', namespace\cents('12.50'), "\n";
echo DEFAULT_LOCALE, ' ', strlen('abc'), ' ', \strlen('abcd'), ' ', PHP_VERSION, "\n";
try {
    $now = new DateTimeImmutable();          // classes never fall back to the global space
} catch (\Error $e) {
    echo $e->getMessage(), "\n";
}

Output

Acme\ShopMoney | Acme\ShopMoney\Money | 1250
en_US 3 4 8.5.4
Class "Acme\ShopMoney\DateTimeImmutable" not found

Names resolve like file paths: Money and Exception\CurrencyMismatch are relative to the current namespace, \DateTimeImmutable is absolute, and names in strings (new $class) are always absolute, without the leading backslash. A missing function or constant falls back to the global one (strlen, PHP_VERSION), but a class never does: write \DateTimeImmutable or import it (Subsection 4.12.3).

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Declaring and Using Namespaces