A brief analysis of how to use PHP to implement a simple calendar program? (with code)

<?php
    class Calendar{
        private $year, $month, $start_week, $days;
        /**
         * 构造方法，用来初始化一些日期属性
         */
        function __construct(){
            $this->year = isset($_GET[&#39;year&#39;])?$_GET[&#39;year&#39;]:date(&#39;Y&#39;);
            $this->month = isset($_GET[&#39;month&#39;])?$_GET[&#39;month&#39;]:date(&#39;m&#39;);
            $this->start_week = date(&#39;w&#39;, mktime(0, 0, 0, $this->month, 1, $this->year));
            $this->days = date(&#39;t&#39;, mktime(0, 0, 0, $this->month, 1, $this->year));
        }
        /**
         * 魔术方法，用来打印整个日历
         * @return string [日历的html代码]
         */
        function __toString(){
            $output = &#39;&#39;;
            $output = &#39;<table>&#39;;
            $output .= $this->changeDate();
            $output .= $this->weeksList();
            $output .= $this->daysList();
            $output .= &#39;</table>&#39;;
            return $output;
        }
        /**
         * 输出周列表
         * @return [string] [html 代码]
         */
        private function weeksList($output=&#39;&#39;){
            $week = array(&#39;日&#39;,&#39;一&#39;,&#39;二&#39;,&#39;三&#39;,&#39;四&#39;,&#39;五&#39;,&#39;六&#39;);
            $output .= &#39;<tr>&#39;;
            for ($i=0; $i < count($week); $i++) {
                $output .= &#39;<th>&#39;.$week[$i].&#39;</th>&#39;;
            }
            $output .= &#39;</tr>&#39;;
            return $output;
        }
        /**
         * 输出日期列表
         * @return [string]
         */
        private function daysList($output=&#39;&#39;){
            $output .= &#39;<tr>&#39;;
            for ($i=0; $i < $this->start_week; $i++) {
                $output .= &#39;<td> </td>&#39;;
            }
            for ($j=1; $j <= $this->days; $j++) {
                $i++;
                if($j == date(&#39;d&#39;) && $this->year == date(&#39;Y&#39;) && $this->month == date(&#39;m&#39;)){
                    $output .= &#39;<td>&#39;.$j.&#39;</td>&#39;;
                }else{
                    $output .= &#39;<td>&#39;.$j.&#39;</td>&#39;;
                }
                if($i%7 == 0) $output .= &#39;</tr><tr>&#39;;
            }
            while($i%7 !== 0){
                $output .= &#39;<td> </td>&#39;;
                $i++;
            }
            $output .= &#39;</tr>&#39;;
            return $output;
        }
        /**
         * 处理上一年的数据
         * @param  [type] $year  [年份]
         * @param  [type] $month [月份]
         * @return [type]        [description]
         */
        private function prevYear($year, $month){
            $year -= 1;
            if($year < 1970) $year = 1970;
            return "year=$year&month=$month";
        }
        /**
         * 处理上一月的数据
         * @param  [type] $year  [年份]
         * @param  [type] $month [月份]
         * @return [type]        [description]
         */
        private function prevMonth($year, $month){
            if($month == 1){
                $year -= 1;
                if($year < 1970) $year = 1970;
                $month = 12;
            }else{
                $month--;
            }
            return "year=$year&month=$month";
        }
        /**
         * 处理下一年的数据
         * @param  [type] $year  [年份]
         * @param  [type] $month [月份]
         * @return [type]        [description]
         */
        private function nextYear($year, $month){
            $year += 1;
            if($year > 2038) $year = 2038;
            return "year=$year&month=$month";
        }
        /**
         * 处理下一月的数据
         * @param  [type] $year  [年份]
         * @param  [type] $month [月份]
         * @return [type]        [description]
         */
        private function nextMonth($year, $month){
            if($month == 12){
                $year --;
                if($year > 2038) $year = 2038;
                $month = 1;
            }else{
                $month++;
            }
            return "year=$year&month=$month";
        }
        /**
         * 调整年份和月份
         * @param  string $output [html代码]
         * @param  string $url   
         * @return [type]        
         */
        private function changeDate($output=&#39;&#39;, $url=&#39;index.php&#39;){
            $output .= &#39;<tr>&#39;;
            $output .= &#39;<td><a href="&#39;.$url.&#39;?&#39;.$this->prevYear($this->year, $this->month).&#39;">&#39;.&#39;<<&#39;.&#39;</a></td>&#39;;
            $output .= &#39;<td><a href="&#39;.$url.&#39;?&#39;.$this->prevMonth($this->year, $this->month).&#39;">&#39;.&#39;<&#39;.&#39;</a></td>&#39;;
            $output .= &#39;<td colspan="3">&#39;;
            $output .= &#39;<form>&#39;;
            $output .= &#39;<select name="year" onchange="window.location=\&#39;&#39;.$url.&#39;?year=\&#39;+this.options[selectedIndex].value+\&#39;&month=&#39;.$this->month.&#39;\&#39;">&#39;;
            for ($i=1970; $i <=2038; $i++) {
                $selected = ($i == $this->year)?&#39;selected="selected"&#39;:&#39;&#39;;
                $output .= &#39;<option value="&#39;.$i.&#39;" &#39;.$selected.&#39;>&#39;.$i.&#39;</option>&#39;;
            }
            $output .= &#39;</select>&#39;;
            $output .= &#39;<select name="month" onchange="window.location=\&#39;&#39;.$url.&#39;?year=&#39;.$this->year.&#39;&month=\&#39;+this.options[selectedIndex].value">&#39;;
            for ($j=1; $j <=12; $j++) {
                $selected = ($j == $this->month)?&#39;selected="selected"&#39;:&#39;&#39;;
                $output .= &#39;<option value="&#39;.$j.&#39;" &#39;.$selected.&#39;>&#39;.$j.&#39;</option>&#39;;
            }
            $output .= &#39;</select>&#39;;
            $output .= &#39;</form>&#39;;
            $output .= &#39;</td>&#39;;
            $output .= &#39;<td><a href="&#39;.$url.&#39;?&#39;.$this->nextMonth($this->year, $this->month).&#39;">&#39;.&#39;>&#39;.&#39;</a></td>&#39;;
            $output .= &#39;<td><a href="&#39;.$url.&#39;?&#39;.$this->nextYear($this->year, $this->month).&#39;">&#39;.&#39;>>&#39;.&#39;</a></td>&#39;;
            $output .= &#39;</tr>&#39;;
            return $output;
        }
    }
?>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>PHP实现简单的日历程序</title>
    <style>
        table{
            border: 1px solid #ccc;
        }
        .fontb{
            color: white;
            background: blue;
        }
        th{
            width: 30px;
        }
        td,th{
            height:30px;
            text-align: center;
        }
        form{
            margin: 0px;
            padding: 0px;
        }
    </style>
</head>
<body>
    <?php
        $calendar = new Calendar;
        echo $calendar;
    ?>
</body>
</html>