PHP namespace

PHP namespace (namespace)

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

PHP namespace can solve the following two types of problems:

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

2. 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 constants, classes and function names are placed in the global space, just like PHP supports naming Space is the same as before.

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

< ?php // 定义代码在 'MyProject' 命名空间中 namespace MyProject; // ... 代码 ...

You can also define different namespace codes in the same file, such as:

< ?php namespace MyProject1; // MyProject1 命名空间中的PHP代码 namespace MyProject2; // MyProject2 命名空间中的PHP代码 // 另一种语法 namespace MyProject3 { // MyProject3 命名空间中的PHP代码 } ?>

The only legal code before declaring a namespace is the one used to define the source file Encoding declare statement. All non-PHP code, including whitespace, must not appear before a namespace declaration.

The following code will cause a syntax error:

 ” 会致命错误 -　命名空间必须是程序脚本的第一条语句 ?>

Subnamespace

with directory Much like files, PHP namespaces also allow you to specify hierarchical namespace names. Therefore, namespace names can be defined in a hierarchical manner:

The above example creates the constant MyProject\Sub\Level\CONNECT_OK, the class MyProject\Sub\Level\Connection and the function MyProject\Sub\Level \Connect.

#Namespace usage

Class names in the PHP namespace can be referenced in three ways:

1. Unqualified names, or class names that do not include a prefix, such as $a=new foo(); or foo: :staticmethod();. If the current namespace is currentnamespace, foo will be resolved to currentnamespace\foo. If the code using foo is global and does not contain code in any namespace, foo will be resolved as foo. Warning: If a function or constant in the namespace is undefined, the unqualified function or constant name is resolved to a global function or constant name.

2. Qualified name, or name containing prefix, such as $a = new subnamespace\foo(); or subnamespace\foo::staticmethod();. If the current namespace is currentnamespace, foo will be resolved to currentnamespace\subnamespace\foo. If the code using foo is global, code not contained in any namespace, foo will be resolved to subnamespace\foo.

3. Fully qualified name, or a name that includes a global prefix operator, for example, $a = new \currentnamespace\foo(); or \currentnamespace\foo::staticmethod();. In this case, foo is always resolved to the literal name currentnamespace\foo in the code.

The following is an example of using these three methods:

file1.php file code

file2.php file code

Note that to access any global class, function or constant, you can use a fully qualified name, such as \strlen() or \Exception or \INI_ALL.

Accessing global classes, functions and constants inside a namespace:

##Namespaces and dynamic language features

The implementation of PHP namespaces is affected by the dynamic characteristics of the language itself. So if you want to convert the code below into a namespace, access the elements dynamically.

example1.php file code:

Must use a fully qualified name (class name including namespace prefix). Note that the leading backslash is unnecessary because there is no difference between qualified and fully qualified names in dynamic class names, function names, or constant names.

Dynamic access to namespace elements

namespace keyword and __NAMESPACE__ Constants

PHP supports two abstract methods of accessing elements within the current namespace, the __NAMESPACE__ magic constant and the namespace keyword.

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.

__NAMESPACE__ example, code in a namespace

__NAMESPACE__ example, global code

The constant __NAMESPACE__ is useful when creating names dynamically, for example:

Use __NAMESPACE__ to dynamically create names

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 classes.

namespace operator, code in the namespace

namespace operator, global code

Use namespace :Alias/Import

PHP namespace support has two ways of using aliases or imports: using aliases for class names, or using aliases for namespace names. Note that PHP does not support imported functions or constants.

In PHP, aliases are implemented through the operator use. The following is an example of using all three possible import methods:

1. Use the use operator to import/use aliases

2. One line contains multiple use statements

The import operation is executed during compilation, but the dynamic class name, function name or constant name is not.

3. Import and dynamic names

In addition, the import operation only affects unqualified names and qualified names. Fully qualified names are not affected by imports because they are deterministic.

4, Import and fully qualified name

Using namespaces: fallback global functions/constants

In a namespace, when PHP encounters an unqualified class, function, or constant name, it uses a different priority strategy to resolve the name. Class names always resolve to names in the current namespace. Therefore, when accessing class names inside the system or not included in the namespace, you must use fully qualified names, for example:

1. Access global classes in the namespace

For functions and For 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.

2. Backup global functions/constants in the namespace

##Global space

If no namespace is defined, all classes and functions are defined in the global space, just like before PHP introduced the namespace concept. Prefixing a name with \ indicates that the name is in the global space, even if the name is in another namespace.

Instructions for using global space

The order of namespaces

Since there is a namespace After that, the most error-prone thing is when using a class, what is the search path for this class.

Name resolution follows the following rules:

1. Calls to functions, classes, and constants with fully qualified names are resolved at compile time. For example new \A\B resolves to class A\B.

2. All unqualified names and qualified names (non-fully qualified names) are converted at compile time according to the current import rules. For example, if namespace A\B\C is imported as C, then calls to C\D\e() will be translated to A\B\C\D\e().

3. Within the namespace, all qualified names that are not converted according to import rules will have the current namespace name in front of them. For example, if C\D\e() is called within namespace A\B, C\D\e() will be converted to A\B\C\D\e().

4. Unqualified class names are converted at compile time according to the current import rules (full names are used instead of short import names). For example, if namespace A\B\C is imported as C, then new C() is converted to new A\B\C() .

5. Within a namespace (e.g. A\B), function calls to unqualified names are resolved at runtime. For example, the call to function foo() is parsed like this:

1. Find the function named A\B\foo() in the current namespace

2. Try to find and call Function foo() in global space.

6. Calls to unqualified names or qualified name classes (non-fully qualified names) within a namespace (e.g. A\B) are resolved at runtime. The following is the parsing process of calling new C() and new D\E(): Parsing of new C():

1. Find classes A\B\C in the current namespace.

2. Try to automatically load class A\B\C.

Parsing of new D\E():

3. Add the current namespace name in front of the class name to become: A\B\D\E, and then search for the class.

4. Try to automatically load class A\B\D\E.

In order to reference a global class in the global namespace, the fully qualified name new \C() must be used.