Detailed explanation of php namespace instances

PHP Namespace(namespace) was added in PHP 5.3. If you have learned C# and Java, then namespace is nothing. New things. However, it still has a very important significance in PHP.

PHP namespace can solve the following two types of problems:

Name conflicts between user-written code and PHP internal classes/functions/constants or third-party classes/functions/constants.

Create an alias (or short) name for a very long identifier name (usually defined to alleviate the first type of problem) to improve the readability of the source code.

Define namespace

By default, all constant, class and function names are placed in the global space, just like before PHP supported namespaces.

The namespace is declared through the keyword namespace. If a file contains a namespace, it must declare the namespace before all other code.

Three ways to reference namespace identifiers

(1) Fully-qualified name

is similar to the absolute path on the operating system, and It is the complete path, so there will be no misunderstanding when understanding it.

For example, in new \A\B\C, then C will be parsed to the C class under the A\B namespace.

(2) Qualified name

is similar to a relative path on the operating system. It contains part of the name and is referenced to the current namespace.

For example, if B\C() is called under namespace A, the finally referenced namespace is A\B\C().

(3) Unqualified name

is similar to Qualified name, but does not include sub-namespaces.

For example, if C() is called under the namespace A\B, the finally referenced namespace is A\B\C().

Illustrate the three citation methods through an example:

namespace \Example;
require_once "fnction.php";
class ClassA {}
function Function() {}
//完全限定名称
\Example\Function();
\Example\B\Function(); 
//限定名称
B\Function(); //指向 \Example\B\Function();
//非限定名称
$test = new ClassA(); //resolves to \Example\ClassA
Function(); //指向 \Example\Function

Note:

• Inside a namespace, if no definitions of functions and constants are found in the current scope, PHP will not report an error. Instead, look for it in the global namespace.

•Inside a namespace, if no class definition is found in the current scope, PHP will report an error directly and will not go to the global scope to find the corresponding class, so if you need to reference an internal or user-defined Class, the fully qualified name must be used.

Let’s take a simple example. First, write a piece of code (defined in the namespace) and name it function.php:

namespace Foo\Bar\subnamespace;
const FOO = 1;
function foo() 
{
  return "foo\r\n";
}
class foo 
{
  static function staticmethod()
  {
    return METHOD . "\r\n" ;
  }
  function foofunction()
  {
    return METHOD . "\r\n" ;
  }
}

Then write a piece of code test.php, which is also in the namespace. The code below:

namespace secondsp;
include 'function.php';
class foo
{
  function foofunction()
  {
    return METHOD . "\r\n" ;
  }
}
function is_file($file)
{
  return true ;
}
//非限定名称:实例化secondsp\foo类对象
$obj = new foo; 
echo $obj->foofunction();
//实例化Foo\Bar\subnamespace\foo 类对象
$obj = new Foo\Bar\subnamespace\foo ;
echo $obj->foofunction();
//代码会报错，在命名空间内部，假如无法找到当前命名空间下的类，则会报错
//$obj = new ArrayObject(array(1)); 
$obj = new \ArrayObject(array(1)); 
//在命名空间内部，假如无法找到当前命名空间下的函数或者常量，则会寻找 native function
echo strlen("nihao");
 //引用当前命名空间下的函数
var_dump(is_file('nihao')); //True
//引用全局函数
var_dump(\is_file('nihao')); //False

Import, alias

If the namespace hierarchy to be used is very long and there are many, it will be particularly troublesome to use, so you can Use the use keyword to import namespaces, classes, constants, functions, etc., which can then be used to reference the full names directly. The alias keyword can rename imported classes and functions.

For example, how to use the use keyword, the code is under the global namespace:

include 'function.php';
use Foo\Bar\subnamespace\foo ;
$obj = new foo;
echo $obj->foofunction();
use Foo\Bar\subnamespace\foo as aliasfunname;
$obj = new aliasfunname;
echo $obj->foofunction();
use Foo\Bar\subnamespace ; 
$obj = new subnamespace\foo ;
echo $obj->foofunction();
use Foo\Bar\subnamespace as aliasname;
$obj = new aliasname\foo ;
echo $obj->foofunction();
//由于调用代码并不在命名空间内，所以对于全局的类，无需引入使用
$obj = new ArrayObject(array(1)); 
//导入一个函数
use function Foo\Bar\subnamespace\foo  ;
echo foo();
use function Foo\Bar\subnamespace\foo as func;
echo func();
use const Foo\Bar\subnamespace\FOO;
//echo FOO;

Summary:

•It is different from Python , The namespace in PHP is a semantic concept and has nothing to do with the location and layout of the specific code. In other words, the code that uses the namespace needs to import the library files (all files) by itself. It does not matter how the library files are organized; In Python, if there is an init.py file in a module or package, the Python parser will automatically import the files of the package or all modules.

•The concept of scope in PHP is very weak, and the global domain and the local domain are clearly distinguished. For example, variables in the global space cannot be referenced in functions or classes. The situation is different in namespaces. If the code that defines the namespace cannot find the constants and functions in the corresponding namespace, it will use global constants and functions; and if it cannot find the class in the corresponding namespace (including its own Define class), the code will report an error directly.

•If you use the namespace through the use keyword, you do not need to import it through the fully qualified name (\ symbol), because PHP already assumes that the imported namespace is a fully qualified namespace.

•Constants, functions, classes, interfaces, and other namespaces can be imported through the use keyword.

•Namespace is a language feature. In order to pursue more effective use, there should be a usage specification and automatic loading mechanism. This is the PSR-4 specification.

The above is the detailed content of Detailed explanation of php namespace instances. For more information, please follow other related articles on the PHP Chinese website!