PHP在类中声明由另一个变量构成的变量

I yesterday discovered strange issue in my class - hope you know some answers. Consider such a class:

class Person {

public $height = 90;
public $weight = $this->height * 0.8;

}

This class returns an error "Parse error: syntax error, unexpected T_VARIABLE" and it seems i can not declare variable in class that is variable itself. Can I only set "static" values to variables in class (i mean static like directly declared like string or int no static like "static $var = 'xyz'";Why is that happening?

Thanks, Kalreg.

Currently, you can use only constant expressions when defining default properties values in PHP. That means you can not use anything that will be evaluated at run-time. Since $this refers to dynamic instance value, it is run-time, obviously, and cannot be used in such definitions.

properties cannot have dynamic values in the class definition, but you can define the weight in the __construct method if you want:

class Person {

  public $height = 90;
  public $weight;

  public function __construct()
  {
    $this->weight = $this->height * 0.8;
  }

}