php链式操作的实现方法

<?php
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
?>

<?php
class String
{
    public $value;

    public function __construct($str=null)
    {
        $this->value = $str;
    }

    public function __call($name, $args)
    {
        $this->value = call_user_func($name, $this->value, $args[0]);
        return $this;
    }

    public function strlen()
    {
        return strlen($this->value);
    }
}
$str = new String('01389');
echo $str->trim('0')->strlen();
// 输出结果为 4；trim('0')后$str为"1389"
?>