What is PHP singleton mode? How to implement singleton mode in php, php mode

1. What is singleton mode?
1. Meaning
As an object creation mode, the singleton mode ensures that a certain class has only one instance, instantiates itself and provides this instance globally to the entire system. It does not create a copy of the instance, but returns a reference to the instance stored inside the singleton class.
2. Three key points of the singleton model:
(1). A static member variable that holds the only instance of the class is required:
private static $_instance;
(2). Constructors and clone functions must be declared private to prevent external programs from creating new classes and thus losing the meaning of the singleton mode:

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

(3). A public static method (usually the getInstance method) must be provided to access this instance, thereby returning a reference to the unique instance

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

2. Why use singleton mode?
1. Disadvantages of PHP: The PHP language is an interpreted scripting language. This operating mechanism allows all related resources to be recycled after each PHP page is interpreted and executed. 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 throughout the life cycle of the application. Variables are cross-page level and can truly make this instance unique in the application life cycle. 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 will be cleared after the page is executed. It seems that PHP The singleton mode is meaningless, 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 resource.

2. Application scenarios of singleton mode in PHP:
(1), application and database interaction There will be a large number of database operations in an application, such as the act of 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 in the system to globally control certain configuration information, it can be easily implemented using the singleton pattern.

3. How to implement singleton mode?
1. Ordinary database access example:

<&#63;php 
...... 
//初始化一個(gè)數(shù)據(jù)庫(kù)句柄 
$db = new DB(...); 
 
//添加用戶信息 
$db->addUserInfo(...); 
 
...... 
 
//在函數(shù)中訪問數(shù)據(jù)庫(kù)，查找用戶信息 
function getUserInfo() 
{ 
  $db = new DB(...);//再次new 數(shù)據(jù)庫(kù)類，和數(shù)據(jù)庫(kù)建立連接 
  $db = query(....);//根據(jù)查詢語句訪問數(shù)據(jù)庫(kù) 
} 
 
&#63;> 

2. Apply singleton mode to operate the database:

<&#63;php 
class DB  
{  
  private $_db;  
  private static $_instance;  
  
  private function __construct(...)  
  {  
    $this->_db = pg_connect(...);//postgrsql  
  }  
  
  private function __clone() {}; //覆蓋__clone()方法，禁止克隆  
  
  public static function getInstance()  
  {  
    if(! (self::$_instance instanceof self) ) {  
      self::$_instance = new self();  
    }  
    return self::$_instance;  
  }  
  
  public function addUserInfo(...) 
  { 
  } 
   public function getUserInfo(...) 
  {  
  } 
 
} 
 
//test  
$db = DB::getInstance();  
$db->addUserInfo(...);  
$db->getUserInfo(...);  
 
&#63;> 

<&#63;php
/**
 * MyPDO
 */
class MyPDO
{
  protected static $_instance = null;
  protected $dbName = '';
  protected $dsn;
  protected $dbh;
  
  /**
   * 構(gòu)造
   * 
   * @return MyPDO
   */
  private function __construct($dbHost, $dbUser, $dbPasswd, $dbName, $dbCharset)
  {
    try {
      $this->dsn = 'mysql:host='.$dbHost.';dbname='.$dbName;
      $this->dbh = new PDO($this->dsn, $dbUser, $dbPasswd);
      $this->dbh->exec('SET character_set_connection='.$dbCharset.', character_set_results='.$dbCharset.', character_set_client=binary');
    } catch (PDOException $e) {
      $this->outputError($e->getMessage());
    }
  }
  
  /**
   * 防止克隆
   * 
   */
  private function __clone() {}
  
  /**
   * Singleton instance
   * 
   * @return Object
   */
  public static function getInstance($dbHost, $dbUser, $dbPasswd, $dbName, $dbCharset)
  {
    if (self::$_instance === null) {
      self::$_instance = new self($dbHost, $dbUser, $dbPasswd, $dbName, $dbCharset);
    }
    return self::$_instance;
  }
  
  /**
   * Query 查詢
   *
   * @param String $strSql SQL語句
   * @param String $queryMode 查詢方式(All or Row)
   * @param Boolean $debug
   * @return Array
   */
  public function query($strSql, $queryMode = 'All', $debug = false)
  {
    if ($debug === true) $this->debug($strSql);
    $recordset = $this->dbh->query($strSql);
    $this->getPDOError();
    if ($recordset) {
      $recordset->setFetchMode(PDO::FETCH_ASSOC);
      if ($queryMode == 'All') {
        $result = $recordset->fetchAll();
      } elseif ($queryMode == 'Row') {
        $result = $recordset->fetch();
      }
    } else {
      $result = null;
    }
    return $result;
  }
  
  /**
   * Update 更新
   *
   * @param String $table 表名
   * @param Array $arrayDataValue 字段與值
   * @param String $where 條件
   * @param Boolean $debug
   * @return Int
   */
  public function update($table, $arrayDataValue, $where = '', $debug = false)
  {
    $this->checkFields($table, $arrayDataValue);
    if ($where) {
      $strSql = '';
      foreach ($arrayDataValue as $key => $value) {
        $strSql .= ", `$key`='$value'";
      }
      $strSql = substr($strSql, 1);
      $strSql = "UPDATE `$table` SET $strSql WHERE $where";
    } else {
      $strSql = "REPLACE INTO `$table` (`".implode('`,`', array_keys($arrayDataValue))."`) VALUES ('".implode("','", $arrayDataValue)."')";
    }
    if ($debug === true) $this->debug($strSql);
    $result = $this->dbh->exec($strSql);
    $this->getPDOError();
    return $result;
  }
  
  /**
   * Insert 插入
   *
   * @param String $table 表名
   * @param Array $arrayDataValue 字段與值
   * @param Boolean $debug
   * @return Int
   */
  public function insert($table, $arrayDataValue, $debug = false)
  {
    $this->checkFields($table, $arrayDataValue);
    $strSql = "INSERT INTO `$table` (`".implode('`,`', array_keys($arrayDataValue))."`) VALUES ('".implode("','", $arrayDataValue)."')";
    if ($debug === true) $this->debug($strSql);
    $result = $this->dbh->exec($strSql);
    $this->getPDOError();
    return $result;
  }
  
  /**
   * Replace 覆蓋方式插入
   *
   * @param String $table 表名
   * @param Array $arrayDataValue 字段與值
   * @param Boolean $debug
   * @return Int
   */
  public function replace($table, $arrayDataValue, $debug = false)
  {
    $this->checkFields($table, $arrayDataValue);
    $strSql = "REPLACE INTO `$table`(`".implode('`,`', array_keys($arrayDataValue))."`) VALUES ('".implode("','", $arrayDataValue)."')";
    if ($debug === true) $this->debug($strSql);
    $result = $this->dbh->exec($strSql);
    $this->getPDOError();
    return $result;
  }
  
  /**
   * Delete 刪除
   *
   * @param String $table 表名
   * @param String $where 條件
   * @param Boolean $debug
   * @return Int
   */
  public function delete($table, $where = '', $debug = false)
  {
    if ($where == '') {
      $this->outputError("'WHERE' is Null");
    } else {
      $strSql = "DELETE FROM `$table` WHERE $where";
      if ($debug === true) $this->debug($strSql);
      $result = $this->dbh->exec($strSql);
      $this->getPDOError();
      return $result;
    }
  }
  
  /**
   * execSql 執(zhí)行SQL語句
   *
   * @param String $strSql
   * @param Boolean $debug
   * @return Int
   */
  public function execSql($strSql, $debug = false)
  {
    if ($debug === true) $this->debug($strSql);
    $result = $this->dbh->exec($strSql);
    $this->getPDOError();
    return $result;
  }
  
  /**
   * 獲取字段最大值
   * 
   * @param string $table 表名
   * @param string $field_name 字段名
   * @param string $where 條件
   */
  public function getMaxValue($table, $field_name, $where = '', $debug = false)
  {
    $strSql = "SELECT MAX(".$field_name.") AS MAX_VALUE FROM $table";
    if ($where != '') $strSql .= " WHERE $where";
    if ($debug === true) $this->debug($strSql);
    $arrTemp = $this->query($strSql, 'Row');
    $maxValue = $arrTemp["MAX_VALUE"];
    if ($maxValue == "" || $maxValue == null) {
      $maxValue = 0;
    }
    return $maxValue;
  }
  
  /**
   * 獲取指定列的數(shù)量
   * 
   * @param string $table
   * @param string $field_name
   * @param string $where
   * @param bool $debug
   * @return int
   */
  public function getCount($table, $field_name, $where = '', $debug = false)
  {
    $strSql = "SELECT COUNT($field_name) AS NUM FROM $table";
    if ($where != '') $strSql .= " WHERE $where";
    if ($debug === true) $this->debug($strSql);
    $arrTemp = $this->query($strSql, 'Row');
    return $arrTemp['NUM'];
  }
  
  /**
   * 獲取表引擎
   * 
   * @param String $dbName 庫(kù)名
   * @param String $tableName 表名
   * @param Boolean $debug
   * @return String
   */
  public function getTableEngine($dbName, $tableName)
  {
    $strSql = "SHOW TABLE STATUS FROM $dbName WHERE Name='".$tableName."'";
    $arrayTableInfo = $this->query($strSql);
    $this->getPDOError();
    return $arrayTableInfo[0]['Engine'];
  }
  
  /**
   * beginTransaction 事務(wù)開始
   */
  private function beginTransaction()
  {
    $this->dbh->beginTransaction();
  }
  
  /**
   * commit 事務(wù)提交
   */
  private function commit()
  {
    $this->dbh->commit();
  }
  
  /**
   * rollback 事務(wù)回滾
   */
  private function rollback()
  {
    $this->dbh->rollback();
  }
  
  /**
   * transaction 通過事務(wù)處理多條SQL語句
   * 調(diào)用前需通過getTableEngine判斷表引擎是否支持事務(wù)
   *
   * @param array $arraySql
   * @return Boolean
   */
  public function execTransaction($arraySql)
  {
    $retval = 1;
    $this->beginTransaction();
    foreach ($arraySql as $strSql) {
      if ($this->execSql($strSql) == 0) $retval = 0;
    }
    if ($retval == 0) {
      $this->rollback();
      return false;
    } else {
      $this->commit();
      return true;
    }
  }
  /**
   * checkFields 檢查指定字段是否在指定數(shù)據(jù)表中存在
   *
   * @param String $table
   * @param array $arrayField
   */
  private function checkFields($table, $arrayFields)
  {
    $fields = $this->getFields($table);
    foreach ($arrayFields as $key => $value) {
      if (!in_array($key, $fields)) {
        $this->outputError("Unknown column `$key` in field list.");
      }
    }
  }
  
  /**
   * getFields 獲取指定數(shù)據(jù)表中的全部字段名
   *
   * @param String $table 表名
   * @return array
   */
  private function getFields($table)
  {
    $fields = array();
    $recordset = $this->dbh->query("SHOW COLUMNS FROM $table");
    $this->getPDOError();
    $recordset->setFetchMode(PDO::FETCH_ASSOC);
    $result = $recordset->fetchAll();
    foreach ($result as $rows) {
      $fields[] = $rows['Field'];
    }
    return $fields;
  }
  
  /**
   * getPDOError 捕獲PDO錯(cuò)誤信息
   */
  private function getPDOError()
  {
    if ($this->dbh->errorCode() != '00000') {
      $arrayError = $this->dbh->errorInfo();
      $this->outputError($arrayError[2]);
    }
  }
  
  /**
   * debug
   * 
   * @param mixed $debuginfo
   */
  private function debug($debuginfo)
  {
    var_dump($debuginfo);
    exit();
  }
  
  /**
   * 輸出錯(cuò)誤信息
   * 
   * @param String $strErrMsg
   */
  private function outputError($strErrMsg)
  {
    throw new Exception('MySQL Error: '.$strErrMsg);
  }
  
  /**
   * destruct 關(guān)閉數(shù)據(jù)庫(kù)連接
   */
  public function destruct()
  {
    $this->dbh = null;
  }
}
&#63;>

<&#63;php
require 'MyPDO.class.php';
$db = MyPDO::getInstance('localhost', 'root', '123456', 'test', 'utf8');
$db->query("select count(*) frome table");
$db->destruct();
&#63;>