Namespaces - PHP Manual Notes

Overview

Namespaces are a way of encapsulating things. This abstract concept can be seen in many places. For example, in operating systems, directories are used to group related files. For files in the directory, the directory plays the role of a namespace. The application of this principle to the field of programming is the concept of namespace. Versions after PHP 5.3 begin to support namespaces.

Define namespace

Namespaces are declared using the keyword namespace. Any legal PHP code can be included in the namespace, but only three types: classes, functions, and constants are affected by the namespace.

The only legal code before declaring a namespace is the declare statement that defines how the source file is encoded. The namespace must be the first statement in the program script, and all non-PHP code, including whitespace, cannot appear before the namespace declaration. The same namespace can be defined in multiple files.

Define sub namespaces

Similar to the relationship between directories and files, PHP also allows you to specify hierarchical namespaces.

Define multiple namespaces in the same file

PHP supports this syntax, but its use is not recommended. There are two syntax forms for defining multiple namespaces in the same file: one is a simple upper and lower list, and the other is the syntax using curly brackets. It is recommended to use the second one.

<code><?php 
declare(encoding = 'UTF-8');
namespace MyProject {
	/* MyProject */
}
namespace {
	/* global */
}</code>

To combine global non-namespace code with code in the namespace, you can only use curly bracket syntax.

Using Namespaces: Basics

To use namespaces, you must understand how PHP knows which elements in the namespace to use.

In the file system, there are three ways to access a file:

• Relative file name form
  
• Relative path name form
  
• Absolute path name form

Similar to the file system, in the PHP namespace, class names can be referenced in three ways. You need to understand the namespace name definition:

• Unqualified name, class name without prefix. An identifier whose name does not contain a namespace separator. For example Foo.
  
• Qualified names, names containing prefixes, identifiers containing namespace separators in their names. For example FooBar.
  
• Fully qualified name, including the name of the global prefix operator. An identifier whose name contains a namespace delimiter and begins with a namespace delimiter. For example FooBar. namespaceFoo is also a fully qualified name.

Namespaces and dynamic language features

The implementation of namespaces is affected by the dynamic characteristics of the language itself, and sometimes fully qualified names must be used.

namespace keywords and __NAMESPACE__constants

The value of the constant __NAMESPACE__ is a string containing the name of the current namespace. In global code, not included in any namespace, it contains an empty string. The constant __NAMESPACE__ is useful when creating names dynamically.

The

keyword namespace can be used to explicitly access elements in the current namespace or sub-namespaces. It is equivalent to the self operator in the class.

Use namespace: alias/import

Allows referencing or importing external fully qualified names through aliases, similar to how you can create symbolic links to other files or directories in Unix-like file systems.

PHP namespace supports two aliases or import methods:

• Use aliases for class names
  
• Use aliases for namespace names

Use the use operator to import or use aliases, supporting multiple use statements in one line. Note that for names in namespaces, leading backslashes are unnecessary and not allowed, because imported names must be fully qualified and will not be resolved relative to the current namespace.

Global space

If no namespace is defined, all classes and functions are defined in the global space. Prefixing the name with indicates that the name is a name in the global space.

Use namespaces: fallback global functions/constants

In a namespace, when PHP encounters an unqualified class, function or constant name, it will use different wire strategies to resolve the name.

Class names always resolve to names in the current namespace, so when accessing class names internal to the system or not contained in a namespace, the fully qualified name must be used.

For functions and constants, if the function or constant does not exist in the current namespace, PHP will fall back to using the function or constant in the global space.

Name resolution rules

The name resolution rules contain a lot of text and require detailed analysis of specific issues.

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