極奇怪的，曉得bug在哪裡，但不知道為何會出現這bug

極奇怪的，知道bug在哪裡，但不知道為何會出現這bug?

PHP code

year=isset($_GET["year"])?$_GET["year"]:date("Y");            $this->month=isset($_GET["month"])?$_GET["month"]:date("n");            $this->daynum=date("t",mktime(0,0,0,$this->month,1,$this->year));            $this->start_day=date("w",mktime(0,0,0,$this->month,1,$this->year));                                }                function out(){            echo '
  
   ';                $this->changecalendar();                $this->daylist();                $this->formset();                            echo '
  ';        }                        function daylist(){            $day=array('日','一','二','三','四','五','六','七');            echo '';            for($i=0;$i<7;$i++){                echo '';                    echo $day[$i];                echo '';            }            echo '';        }                        function formset() {            echo '';            for($j=0; $j<$this->start_day; $j++)                echo ' ';                                                $k=$this->start_day;            for($j=1;$j<=$this->daynum;$j++){                if($j==date("j")){                    echo ''.$j.'';                }else{                    echo ''.$j.'';                }                $k++;                if($k%7==0)                    echo '';                        }        }                        private function changecalendar(){            echo '';//這個正常            echo 'prevYear($this->year,$this->month).'">'.'<<'.'';//這個不正常            echo 'preYear().'">'.'<<'.'';            echo ''.$this->year.'-'.$this->month.'';                        echo '';        }                private function prevYear($year, $month){            $year=$year-1;                        if($year < 1970)                $year = 1970;            return "year={$year}&month={$month}";            }                        private function preYear(){            $this->year=$this->year-1;                        if($this->year<1970){                $this->year=1970;            }            return "year={$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;                            }            }
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