Take you to play with PHP namespaces

* Namespace

* Let’s first understand an important concept: code reuse

* What is our ultimate goal in learning object-oriented programming? "Code reuse"

* 1. Thoroughly understand the code reuse mechanism?

* **So far, what we have learned about the implementation methods of code reuse

* 1. Function: Code reuse is the most important The simple way;

* 2. Classes and objects: encapsulate functions and the data they may use;

* 3. Traits and interfaces: a special class, the essence Is it still an encapsulation of functions and data;

* 4. Constants: Because constants cannot be modified once defined, they can be declared once and called anywhere without changing the value

* 2. Thoroughly understand what is the global?

* 1. The global and other refer to the current script, which cannot be cross-script, let alone the entire project

* 2. The global and the local are the same For twin brothers, they have a common parent: scope, which controls them

* 3. Thoroughly understand what members are in the php script?

* 1. Variables: include System preset variables and user-defined variables;

* 2. Constants: including system preset constants and user-defined constants;

* 3. Functions: including anonymous functions/function expressions /Self-executing function;

* 4. Classes and objects: including abstract classes, interfaces, traits;

* 3. Thoroughly understand what scope is?

** ****Scope is mainly for variables

*****Variables in PHP have three scopes: function scope, global scope, super global scope;

* 1. Global scope: declared outside the function, cannot be used directly inside the function

* Note: Global variables used in the function must be declared with global, or use a super-global array: $GLOBALS

* 2. Function scope: that is, local scope, including function parameters and variables declared within the function, which are automatically cleared when leaving the function;

* 3. Super global scope: In addition to being globally valid, You can also use it directly within a function without declaring

* 4. Why use namespace?

* 1. Classes, functions, and constants are all globally valid, and they can also be used directly when included in a function Use .

* 2. Therefore, classes, functions and constants with duplicate names are not allowed globally.

* 3. If you want to import a function library or class library from the outside, their function names and class names are likely to conflict with the current script

* 4. Therefore, you must have a set A mechanism for handling identifier naming conflicts for script members working in the global scope.

* 5. Namespace is such a solution. It not only allows users to give global members a shorter name, but also solves

* The introduction of third-party resources brings The problem of duplicate names.

* 6. Without using any namespace, all classes, constants, and functions are declared in the global space, using \ tables. Now we add

* before them. Except for the slash \, you will find that the execution result is also correct.

* When declaring classes, constants, and functions, you do not need to add \ in front of it. You only need to add \

## when calling. # * Namespace is very important. If you cannot fully master it, it will become the biggest obstacle to your future learning!!!