?php
header("Content-type:text/html;Charset=utf-8");
class time{
 private $year;//年
 private $month;//月
 private $day;//天
 private $hour;//小时
 private $minute;//分钟
 private $second;//秒
 private $microtime;//毫秒
 private $weekday;//星期
 private $longDate;//完整的时间格式
 private $diffTime;//两个时间的差值
 //返回年份 time：时间格式为时间 2018-8-21
 function getyear($time="",$type=""){
 if($time==""){
 $time=time();
 }
 if($type==1){
 return $this->year=date("y",$time); //返回两位的年份 18
 }else{
 return $this->year=date("Y",$time); //返回四位的年份 2018
 }
 }
 //返回当前时间的月份 time：时间格式为时间 2018-8-21
 function getmonth($time="",$type=""){
 if($time==""){
 $time=time();
 }
 switch($type){
 case 1:$this->month=date("n",$time);//返回格式 8
  break;
 case 2:$this->month=date("m",$time);//返回格式 08
  break;
 case 3:$this->month=date("M",$time);//返回格式 Aug
  break;
 case 4:$this->month=date("F",$time);//返回格式 August
  break;
 default:$this->month=date("n",$time);
 }
 return $this->month; 
 }
 //返回当前时间的天数 time：时间格式为时间 2018-8-21 
 function getday($time="",$type=""){
 if($time==""){
 $time=time();
 }
 if($type==1){
 $this->day=date("d",$time);//返回格式 21
 }else{
 $this->day=date("j",$time);//返回格式 21
 }
 return $this->day;
 }
 //返回当前时间的小时 2018-08-21 1:19:21 20:19:21 
 function gethour($time="",$type=""){
 if($time==""){
 $time=time();
 } 
 switch($type){
 case 1:$this->hour=date("H",$time);//格式： 1 20
  break;
 case 2:$this->hour=date("h",$time);//格式 01 08
  break;
 case 3:$this->hour=date("G",$time);//格式 1 20
  break;
 case 4:$this->hour=date("g",$time);//格式 1 8
  break; 
 default :$this->hour=date("H",$time);
 }
 return $this->hour;
 }
 //返回当前时间的分钟数 1:9:18 
 function getminute($time="",$type=""){
 if($time==""){
 $time=time();
 }
 $this->minute=date("i",$time); //格式 09
 return $this->minute;
 }
 //返回当前时间的秒数 20:19:01
 function getsecond($time="",$type=""){
 if($time==""){
 $time=time();
 }
 $this->second=date("s",$time); //格式 01
 return $this->second;
 }
 //返回当前时间的星期数 
 function getweekday($time="",$type=""){
 if($time==""){
 $time=time(); 
 }
 if($type==1){
 $this->weekday=date("D",$time);//格式 Sun
 }else if($type==2){
 $this->weekday=date("l",$time); //格式 Sunday
 }else{
 $this->weekday=date("w",$time);//格式 数字表示 0--6
 }
 return $this->weekday;
 }
 //比较两个时间的大小 格式 2018-8-21 8:4:3 
 function compare($time1,$time2){
 $time1=strtotime($time1);
 $time2=strtotime($time2);
 if($time1>=$time2){ //第一个时间大于等于第二个时间 返回1 否则返回0
 return 1;
 }else{
 return -1;
 }
 }
 //比较两个时间的差值
 function diffdate($time1="",$time2=""){
 //echo $time1.'------'.$time2.'br>';
 if($time1==""){
 $time1=date("Y-m-d H:i:s"); 
 }
 if($time2==""){ 
 $time2=date("Y-m-d H:i:s"); 
 }
 $date1=strtotime($time1);
 $date2=strtotime($time2);
 if($date1>$date2){
 $diff=$date1-$date2; 
 }else{
 $diff=$date2-$date1;
 }
 if($diff>=0){
 $day=floor($diff/86400);
 $hour=floor(($diff%86400)/3600);
 $minute=floor(($diff%3600)/60);
 $second=floor(($diff%60));
 $this->diffTime='相差'.$day.'天'.$hour.'小时'.$minute.'分钟'.$second.'秒'; 
 }
 return $this->diffTime;
 }
 //返回 X年X月X日
 function buildDate($time="",$type=""){
 if($type==1){  
 $this->longDate = $this->getyear($time) . '年' . $this->getmonth($time) . '月' . $this->getday($time) . '日'; 
 }else{
 $this->longDate = $this->getyear($time) . '年' . $this->getmonth($time) . '月' . $this->getday($time) . '日'.$this->gethour($time).':'.$this->getminute($time).':'.$this->getsecond($time); 
 }
 return $this->longDate; 
 }
}
?>

?php
  $time_var = "2018-08-21";
  $obj = new time();
  $year = $obj->getyear($time_var);

  echo($year);
?>