单例模式

1．只能有一个实例。

2．必须自行创建这个实例。

3．必须给其他对象提供这一实例

<?php
/**
* Created by PhpStorm.
* User: ASUS
* Date: 2019/4/6
* Time: 19:18
*/

class DB
{
private $_pdo;
private $_table;
private $_field = "*";
private $_where = "";
private $_order;
private $_join;
private $_sql;
private function __construct()
{
$this->_pdo = new PDO("mysql:host=127.0.0.1;dbname=pmp",'root','root');
}
private function __clone()
{
// TODO: Implement __clone() method.
}

private static $_class;
public static function getClass(){
if(self::$_class==''){
self::$_class = new DB();
}
return self::$_class;
}

/*
* 设置表名
* @param string $table 表名
* */
public function table(string $table){
$this->_table = $table;
$this->_sql = "$table";
return $this;
}

/*
* 设置查询的字段
* @param string $fiele 字段名
* */
public function field(string $fiele){
$this->_field = $fiele;
return $this;
}

/*
* 设置查询条件
* @param string $where 查询条件
* */
public function where(string $where){
$this->_where = $where;
$this->_sql.=" where $where";
return $this;
}

/*
* 设置排序查询
* @param string $field 字段名
* @param string $order 排序方式，默认正序
* */
public function order(string $field,string $order = 'asc'){
$this->_order = "order by $field $order";
$this->_sql.=" order by $field $order";
return $this;
}

/*
* 设置分组查询条件
* @param string $group 分组查询条件
* */
public function group(string $group){
$this->_sql.=" GROUP BY $group";
return $this;
}

/*
* 设置分页查询条件
* @param int $offset 偏移量
* @param int $limit 每页条数
* */
public function limit(int $offset,int $limit){
$this->_sql.=" limit $offset,$limit";
return $this;
}

/*
* 关联查询，参数依次为：关联表名、关联条件、链接方式（默认为inner，可传入left|right）
* @param string $table 关联表明
* @param string $join 关联条件
* @param string $dir 链接方式（默认为inner，可传入left|right）
* */
public function join(string $table,string $join,string $dir='inner'){
$table2 = substr($table,0,1);
$this->_join.=" $dir JOIN $table as $table2 ON $join";
return $this;
}

/*
* 查询多条数据，返回二维数组
* @return array 二维数组
* */
public function select(string $sql=''){

if(!empty($sql)){
$this->_sql = $sql;
}else{
if(!empty($this->_join)){
$table1 = substr($this->_table,0,1);
$this->_sql = str_replace("$this->_table","",$this->_sql);
$this->_sql = "select $this->_field from ".$this->_table." as $table1 ".$this->_join." $this->_sql";
}else{
$this->_sql = "select $this->_field from ".$this->_sql;
}
}
return $this->_pdo->query($this->_sql)->fetchAll(PDO::FETCH_ASSOC);
}

/*
* 查询单条数据，返回一维数组
* @return array 一维数组
* */
public function find(){
if(!empty($this->_join)){
$table1 = substr($this->_table,0,1);
$this->_sql = str_replace("$this->_table","",$this->_sql);
$this->_sql = "select $this->_field from ".$this->_table." as $table1 ".$this->_join." $this->_sql";
}else{
$this->_sql = "select $this->_field from ".$this->_sql;
}
// return $this->_sql;
return $this->_pdo->query($this->_sql)->fetch(PDO::FETCH_ASSOC);
}

/*
* 添加单条数据，返回影响记录数，参数为key=>value 格式的一维数组
* @param array $data 添加的数组
* @return int 影响记录的行数
* */
public function add(array $data){
$sql = "insert into $this->_table set ";
foreach ($data as $key=>$value){
$sql.="$key='$value',";
}
return $this->_pdo->exec(substr($sql,0,-1));
}

/*
* 删除数据，返回影响记录数
* @return int 影响记录的行数
* */
public function delete(string $sql=''){
if(empty($sql)){
$sql = "delete from $this->_table WHERE $this->_where";
}
return $this->_pdo->exec($sql);
}

/*
* 修改数据，返回影响记录数，参数为key=>value格式的一维数组
* @param array $data 修改的数组
* @return int 影响记录的行数
* */
public function update(array $data){
$sql = "update $this->_table set ";
foreach ($data as $key=>$value){
$sql.="$key='$value',";
}
$sql = substr($sql,0,-1);
$sql.=" where $this->_where";
return $this->_pdo->exec($sql);
}
}
$obj = DB::getClass();
$data = $obj->table("city")->field("id,name")->where("id<5")->order("id",'desc')->select();
//var_dump($data);

//var_dump($obj->table("city")->add(['name'=>'陕西','pid'=>0]));
//var_dump($obj->table("city")->where("id in (34,36)")->delete());
//var_dump($obj->table("city")->where("id=33")->update(['name'=>'陕西','pid'=>0]));
//var_dump($obj->table("city")->field("c.name,u.id")->join("user","c.id=u.id")->join("score","s.s_id=c.id")->where("u.id=1")->select());
//var_dump($obj->table("score")->group("s_id")->limit(2,3)->select());