极奇怪的，晓得bug在哪里，但不知道为何会出现这bug

极奇怪的，知道bug在哪里，但不知道为何会出现这bug?

PHP code

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<?php class calendar{
        private $year;
        private $month;
        private $daynum;
        private $start_day;
        
        function __construct(){
            $this->year=isset($_GET["year"])?$_GET["year"]:date("Y");
            $this-&gt;month=isset($_GET["month"])?$_GET["month"]:date("n");
            $this-&gt;daynum=date("t",mktime(0,0,0,$this-&gt;month,1,$this-&gt;year));
            $this-&gt;start_day=date("w",mktime(0,0,0,$this-&gt;month,1,$this-&gt;year));
            
            
        }
        
        function out(){
            echo '

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'; $this->changecalendar(); $this->daylist(); $this->formset(); echo ''; } function daylist(){ $day=array('日','一','二','三','四','五','六','七'); echo ''; for($i=0;$i'; echo $day[$i]; echo ''; } echo ''; } function formset() { echo ''; for($j=0; $jstart_day; $j++) echo ' '; $k=$this->start_day; for($j=1;$jdaynum;$j++){ if($j==date("j")){ echo ''.$j.''; }else{ echo ''.$j.''; } $k++; if($k%7==0) echo ' '; } } private function changecalendar(){ echo ' '; //这个正常 echo ''.''; //这个不正常 echo ''.''; echo ''.$this->year.'-'.$this->month.''; echo ''; } private function prevYear($year, $month){ $year=$year-1; if($year year=$this->year-1; if($this->yearyear=1970; } return "year={$this->year}&month={$this->month}"; } private function preMonth(){ if($this->month == 1){ $this->year=$this->year - 1; $this->month=12; }else{ $this->month=$this->month-1; } echo "111"; return 'year='.$this->year.'&month='.$this->month; } private function nextYear(){ $this->year=$this->year + 1; if($this->year>2038){ $this->year=2038; } return 'year='.$this->year.'&month='.$this->month; } private function nextMonth(){ if($this->month==12){ $this->month=1; $this->year=$this->year + 1; }else{ $this->month=$this->month + 1; } return 'year='.$this->year.'&month='.$this->month; } }