1. $property1//具体变量
2. $this->property1//具体属性

1. $property1 // specific variable
2. $this->property1 // specific attribute

类的一般用途是不使用 "$" 否则你正在调用一个名为 $property1 的变量，它可以接受任何值.

The general use on classes is without "$" otherwise you are calling a variable called $property1 that could take any value.

示例:

class X {
  public $property1 = 'Value 1';
  public $property2 = 'Value 2';
}
$property1 = 'property2';  //Name of attribute 2
$x_object = new X();
echo $x_object->property1; //Return 'Value 1'
echo $x_object->$property1; //Return 'Value 2'