Question

When I try to do the following I get a syntax error, unexpected T_VARIABLE. What am I doing wrong?

class myObj {
  public $birth_month;
  public $birthday = array('input_val' => $this->birth_month);
}

I also tried

class myObj {
  public $birth_month;
  public $birthday = array('input_val' => $birth_month);
}

Answer 1

You cannot use an expression to initialize a class property. It must be a constant value, or you must initialize it in the constructor. That’s the source of your syntax error.

class myObj {
  public $birth_month;
  public $birthday;

  // Initialize it in the constructor
  public function __construct($birth_month) {
    $this->birth_month = $birth_month;
    $this->birthday = array('input_val' => $this->birth_month);
  }
}

From the docs on class properties:

They are defined by using one of the keywords public, protected, or private, followed by a normal variable declaration. This declaration may include an initialization, but this initialization must be a constant value–that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

In your first attempt, using $this outside an instance method would not have been supported even baring the compile-time limitation of property initialization, since $this is only meaningful inside instance methods.