PHP Single example design mode connection database detailed

This article mainly and everyone to share PHP single example design mode connection database in detail, mainly in the form of code and we share, hope to help everyone.

<?php    /**     *php Object-oriented: single-state design mode (connection database)     */    class db{        private static $obj =null;  Declare some information about a private database object        //database connection private        function __construct () {            echo "connection to database succeeded";        }        Returns the database connection object (static method)        static function getinstance () {            if (Is_null (self:: $obj)) {  //database object does not exist when self                :: $obj =new self ();  Instantiation            }            return self:: $obj;  Return database Object        }    }    $db =db::getinstance ();  Database Object    $db->query ("select * from ' Student '");

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