PHP类连贯操作的实现方法

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/* * SQL语句组合实例类，始发文章web开发笔记 * www.chhua.com * 学习用，非专业类 * */class sql{	private $sql=array("from"=>"",			"where"=>"",			"order"=>"",			"limit"=>""); 	public function from($tableName) {		$this->sql["from"]="FROM ".$tableName;		return $this;	} 	public function where($_where='1=1') {		$this->sql["where"]="WHERE ".$_where;		return $this;	} 	public function order($_order='id DESC') {		$this->sql["order"]="ORDER BY ".$_order;		return $this;	} 	public function limit($_limit='30') {		$this->sql["limit"]="LIMIT 0,".$_limit;		return $this;	}	public function select($_select='*') {		return "SELECT ".$_select." ".(implode(" ",$this->sql));	}} $sql =new sql(); echo $sql->from("testTable")->where("id=1")->order("id DESC")->limit(10)->select();//输出 SELECT * FROM testTable WHERE id=1 ORDER BY id DESC LIMIT 0,10