Why should php use singleton mode?

The reason why PHP uses singleton mode: When developing in an object-oriented way, using singleton mode can avoid excessive new operations that consume a lot of resources. The singleton mode can globally control certain configuration information through a class.

1. What is the singleton pattern?

1. Meaning

As an object creation mode, the singleton mode ensures that there is only one instance of a certain class, and it instantiates itself and provides it to the entire system. Provide this example. It does not create a copy of the instance, but returns a reference to the instance stored inside the singleton class.

The application of PHP is mainly in database applications, so there will be a large number of database operations in an application. When developing in an object-oriented way, if you use the singleton mode, you can avoid a large number of resources consumed by new operations. . If a class is needed in the system to globally control certain configuration information, it can be easily implemented using the singleton pattern.

2. Three key points of singleton mode:

(1). A static member variable is required to save the only instance of the class:

private static $_instance;

(2). Constructors and clone functions must be declared private to prevent external programs from new classes and thus lose the meaning of the singleton mode:

private function __construct()   
{   
    $this->_db = pg_connect('xxxx');  
}   
private function __clone()  
{  
}//覆盖__clone()方法，禁止克隆

(3). A method to access this instance must be provided Public static method (usually getInstance method), thus returning a reference to the only instance

public static function getInstance()    
{    
    if(! (self::$_instance instanceof self) )   
    {    
        self::$_instance = new self();    
    }  
    return self::$_instance;    
  
}

2. Why use singleton mode?

Most people understand its purpose from the literal meaning of the singleton pattern, thinking that it saves system resources and avoids repeated instantiation. It is a " family planning".

And PHP will clean up all resources from memory every time it executes the page. Therefore, the singleton in PHP actually needs to be re-instantiated every time it is run, thus losing the meaning of repeated instantiation of the singleton.

From this aspect alone, PHP's singleton is indeed a bit disappointing. But does a singleton only have this function and application? The answer is no, let's take a look.

The application of php is mainly in database applications, so there will be a large number of database operations in an application. When developing in an object-oriented way (nonsense), if you use the singleton mode, you can avoid a large number of operations. The resources consumed by the new operation.

If a class is needed in the system to globally control certain configuration information, then it can be easily implemented using the singleton mode. This can be found in the FrontController part of zend Framework.

In a page request, it is easy to debug because all the code (such as database operation class db) is concentrated in one class. We can set hooks in the class and output logs to avoid var_dump everywhere. echo.

1. Disadvantages of PHP:

PHP language is an interpreted scripting language. This operating mechanism allows each PHP page to be interpreted and executed. Related resources will be recycled.

In other words, PHP has no way to make an object resident in memory at the language level. This is different from compiled types such as asp.net and Java. For example, in Java, a singleton will always exist in the entire application. In the life cycle of the program, variables are cross-page level, which can truly make this instance unique in the life cycle of the application.

However, in PHP, all variables, whether they are global variables or static members of the class, are page-level. Every time the page is executed, a new object will be re-established, and it will be deleted after the page is executed. Being cleared, it seems that the PHP singleton mode has no meaning.

So I think the PHP singleton mode is very meaningful only when multiple application scenarios occur in a single page-level request and need to share the same object resources.

2. Application scenarios of singleton mode in PHP:

(1) Interaction between application and database

Will exist in an application For a large number of database operations, such as connecting to the database through a database handle, using the singleton mode can avoid a large number of new operations, because each new operation consumes memory resources and system resources.

(2). Control configuration information

If a class is needed to globally control certain configuration information in the system, it can be easily implemented using the singleton mode.

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