Learn PHP design pattern singleton pattern

/**
* PHP design pattern singleton mode
* $_instance must be declared as a static private variable
* The constructor and destructor must be declared private to prevent external programs from creating new
* classes and thereby losing the meaning of the singleton mode
* getInstance The () method must be set to public, and this method must be called
* to return a reference to the instance
*:: The operator can only access static variables and static functions
* new objects will consume memory
* Usage scenarios: the most commonly used The place is the database connection.
* After using the singleton pattern to generate an object,
* the object can be used by many other objects.
* @link http://bbs.it-home.org
*/
class Danli {
//Static member variables that save class instances
private static $ _instance;
//Construction method of private tag
private function __construct(){
echo 'This is a Constructed method;';
}

//Create __clone method to prevent the object from being copied and cloned
public function __clone(){
trigger_error('Clone is not allow!',E_USER_ERROR);
}
//Single instance method, a public static method used to access the instance
public static function getInstance(){
 if(!(self::$_instance instanceof self)){
self::$_instance = new self;
}
return self::$_instance;
}
public function test(){
echo 'Calling method successfully';
}
}
//Classes that use new to instantiate the private mark constructor will report an error
// $danli = new Danli();
//The correct method is to use the double colon::operator to access the static method to obtain the instance
$danli = Danli::getInstance();
$danli->test();
// Copying (cloning) the object will result in an E_USER_ERROR
$danli_clone = clone $danli;
?>