类：

class ParentClass{
        public $a=10;
        public function setVal($val){

            $this->a = $val;
        }

       public function getVal(){

            return $this->a;
      }
}

实例化

$obj = new ParentClass;
echo $obj->a."<br>";
$obj->setVal(10000);
echo $obj->getVal();

构造函数__construct() 当初始化实例的时候会被触发

class obj{
  public function __construct(){
      echo "the class".__class__."was initialized !!!";
    }
}

new obj;

__destruct() 当实例对象被注销的时候，会自动触发

class obj{
  public function __construct(){
      echo "the class".__class__."was initialized !!!";
    }
  
  public function __destruct(){
    
     echo "Happy Ending !!!";
    }
}

$a = new obj;
unset($a);

extends 子类继承父类

class childOne extends ParentClass{
    
}
new childOne;

当子类存在的变量或则构造函数和父类的相同的时候，会覆盖父类的变量和方法
如果需要保留父类的构造函数同时触发的话，可以使用parent::__construct();

class childOne extends parentClass{
    public function __construct(){
        parent::__construct();
        echo "this is my construct!!!";
    }
}

new childOne;

访问标识符
public 变量，方法 所有的类都可以访问
protected 变量，方法 该类内部访问或则子类通过方法向上继承的方式才可以访问，子类不可以直接去访问
private 变量，方法 只允许该类自己内部访问

class ParentClass{
  public $a =2000;
  public function __construct(){
      echo "the class".__class__."was initialized !!!";
    }
  
  public function __destruct(){
    
     echo "Happy Ending !!!";
    }
  
  protected function getVal(){
     return $this->a;
    }
}
class childOne extends parentClass{
    public function __construct(){
        parent::__construct();
        echo "this is my construct!!!";
    }
}

$ch = new childOne;
echo $ch->getVal();

class ParentClass{
  public $a =2000;
  public function __construct(){
      echo "the class".__class__."was initialized !!!";
    }
  
  public function __destruct(){
    
     echo "Happy Ending !!!";
    }
  
  protected function getVal(){
     return $this->a;
    }
}
class childOne extends ParentClass{
     public function abcd(){
         return $this->getVal();
            
    }
}

$child123 = new childOne;
echo $child123->abcd();

静态变量，静态方法，不需要实例化就可以直接使用的

<?php
// Your code here!
class demo{
   public static $count=10;
   public static function plusOne()
  {
      return "The count is " . ++self::$count . ".<br />";
  }

}
echo demo::$count."<br>";  //注意，这里是有$符号的！！！
echo demo::plusOne();
?>

Reference:
https://code.tutsplus.com/tutorials/object-oriented-php-for-beginners--net-12762