本文实例讲述了PHP单例模式用法。分享给大家供大家参考，具体如下：

<?php
class db {
  public $conn;
  public static $sql;
  public static $instance=null;
  private function __construct(){
    require_once("db.config.php");
    $this->conn = mysql_connect($db["host"],$db["user"],$db["password"]);
    if(!mysql_select_db($db["database"],$this->conn)){
      echo "失败";
    };
    mysql_query("set names utf8",$this->conn);
  }
  public static function getInstance(){
  if(is_null(self::$instance)){
    self::$instance = new self();
  }
    return self::$instance;
  }
  /**
  * 查询数据库
  */
  public function select($table,$condition=array(),$field = array()){
    $where="";
    if(!empty($condition)){
      foreach($condition as $k=>$v){
        $where.=$k."="".$v."" and ";
      }
      $where="where ".$where ."1=1";
    }
    $fieldstr = "";
    if(!empty($field)){
      foreach($field as $k=>$v){
        $fieldstr.= $v.",";
      }
      $fieldstr = rtrim($fieldstr,",");
    } else {
      $fieldstr = "*";
    }
    self::$sql = "select {$fieldstr} from {$table} {$where}";
    $result=mysql_query(self::$sql,$this->conn);
    $resuleRow = array();
    $i = 0;
    while($row=mysql_fetch_assoc($result)){
      foreach($row as $k=>$v){
        $resuleRow[$i][$k] = $v;
      }
    $i++;
    }
    return $resuleRow;
  }
  //添加一条记录
  public function insert($table,$data) {
    $values = "";
    $data = "";
    foreach ($data as $k=>$v) {
      $values .= $k.",";
      $datas .= ""$v"".",";
    }
    $values = rtrim($values,",");
    $datas = rtrim($datas,",");
    self::$sql = "INSERT INTO {$table} ({$values}) VALUES ({$datas})";
    if(mysql_query(self::$sql)) {
      return mysql_insert_id();
    } else {
      return false;
    }
  }
  //修改一条记录
  public function update($table,$data,$condition=array()){
    $where="";
    if(!empty($condition)) {
    foreach($condition as $k=>$v) {
      $where.=$k."="".$v."" and ";
    }
      $where="where ".$where ."1=1";
    }
    $updatastr = "";
    if(!empty($data)) {
    foreach($data as $k=>$v) {
      $updatastr.= $k."="".$v."",";
    }
      $updatastr = "set ".rtrim($updatastr,",");
    }
    self::$sql = "update {$table} {$updatastr} {$where}";
    return mysql_query(self::$sql);
  }
  //删除记录
  public function delete($table,$condition) {
    $where="";
    if(!empty($condition)) {
      foreach($condition as $k=>$v) {
        $where.=$k."="".$v."" and ";
      }
      $where="where ".$where ."1=1";
    }
    self::$sql = "delete from {$table} {$where}";
    return mysql_query(self::$sql);
  }
  public static function getLastSql() {
    echo self::$sql;
  }
}
$db = db::getInstance();
//$list = $db->select("demo",array("name"=>"tom","password"=>"ds"),array("name","password"));
//echo $db->insert("demo",array("name"=>"最近你啦","password"=>"123"));
//echo $db->update("demo",array("name"=>"xxx","password"=>"123"),array("id"=>1));
echo $db->delete("demo",array("id"=>"2"));
db::getLastSql();
echo "<pre>";
?>

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希望本文所述对大家PHP程序设计有所帮助。