<?php  
class user 
{  
 private static $count = 0 ; //记录所有用户的登录情况.  
 public function __construct() {  
  self::$count = self::$count + 1;  
 }  
 public function getcount() {  
  return self::$count;  
 }  
 public function __destruct() {  
  self::$count = self::$count - 1;  
 }  
}  
$user1 = new user();  
$user2 = new user();  
$user3 = new user();  
echo "now here have " . $user1->getcount() . " user";  
echo "<br />";  
unset($user3);  
echo "now here have " . $user1->getcount() . " user";  
?> 

<?php  
class math 
{  
 public static $pi = 3.14;  
}  
// 求一个半径3的园的面积。  
$r = 3;  
echo "半径是 $r 的面积是<br />";  
echo math::$pi * $r * $r;  
echo "<br /><br />";  
//这里我觉得 3.14 不够精确，我把它设置的更精确。  
math::$pi = 3.141592653589793;  
echo "半径是 $r 的面积是<br />";  
echo math::$pi * $r * $r;  
?> 

<?php  
class math 
{  
 public static function max($num1, $num2) {  
  return $num1 > $num2 ? $num1 : $num2;  
 }   
}  
$a = 99;  
$b = 88;  
echo "显示 $a 和 $b 中的最大值是";  
echo "<br />";  
echo math::max($a, $b);  
echo "<br />"; 
echo "<br />"; 
echo "<br />";  
$a = 99;  
$b = 100;  
echo "显示 $a 和 $b 中的最大值是";  
echo "<br />";  
echo math::max($a,$b);  
?> 

<?php  
// 实现最大值比较的math类。  
class math 
{  
 public static function max($num1, $num2) {  
  return $num1 > $num2 ? $num1 : $num2;  
 }  
 public static function max3($num1, $num2, $num3) {  
  $num1 = self::max($num1, $num2);  
  $num2 = self::max($num2, $num3);  
  $num1 = self::max($num1, $num2);    
  return $num1;  
 }  
}  
$a = 99;  
$b = 77;  
$c = 88;  
echo "显示 $a $b $c 中的最大值是";  
echo "<br />";  
echo math::max3($a, $b, $c);  
?>

<?php  
//  
class circle 
{  
 public static $pi = 3.14;  
 public static function circleacreage($r) {  
  return $r * $r * self::$pi;  
 }  
}  
$r = 3;  
echo " 半径 $r 的圆的面积是 " . circle::circleacreage($r);  
?> 

<?php  
// 这个方式是错误的  
class circle 
{  
 public $pi = 3.14;  
 public static function circleacreage($r) {  
  return $r * $r * self::pi;  
 }  
}  
$r = 3;  
echo " 半径 $r 的圆的面积是 " . circle::circleacreage($r);  
?> 

<?php 
// 实现最大值比较的math类。  
class math 
{   
 public function max($num1, $num2) {  
  echo "bad<br />";    
  return $num1 > $num2 ? $num1 : $num2;  
 }  
 public static function max3($num1, $num2, $num3) {  
  $num1 = $this->max($num1, $num2);  
  $num2 = $this->max($num2, $num3);  
  $num1 = $this->max($num1, $num2);    
  return $num1;  
 }  
}  
$a = 99;  
$b = 77;  
$c = 188;  
echo "显示 $a $b $c 中的最大值是";  
echo "<br />";  
echo math::max3($a, $b, $c); //同样的这个会报错 
?> 

<?php  
// 实现最大值比较的math类。  
class math 
{   
 public function max($num1, $num2) {   
  return $num1 > $num2 ? $num1 : $num2;  
 }  
 public static function max3($num1, $num2, $num3) {  
  $num1 = self::max($num1, $num2);  
  $num2 = self::max($num2, $num3);  
  $num1 = self::max($num1, $num2);    
  return $num1;  
 }  
}  
$a = 99;  
$b = 77;  
$c = 188;  
echo "显示 $a $b $c 中的最大值是";  
echo "<br />";  
echo math::max3($a, $b, $c);  
?>