Why set variables inside the construct of a PHP class when you can set them when they are declared?

Question

Is there a reason to set a value for variables in the constructor of a class rather than when you declare them? I realize you can't pass data into the variables if you try to set them when they are declared, but what about things that will always be the same ( i suppse 'always' is a tough claim to make )?

class variables_set_outside_constructor
{
     private $admin_name = 'jeff';
     private $current_working_directory = get_cwd();

     function __construct() {}
}

as opposed to this:

class variables_set_inside_constructor
{
     private $admin_name;
     private $current_working_directory;

    function __construct()
    {
         $this->admin_name = 'jeff';
         $this->current_working_directory = get_cwd();
    }
}

What are the advantages and disadvantages of settings values in the constructor versus when they are declared? I'm curious about any language agnostic aspects as well.

Answer 1

You have an error in your question. This does not work in a class:

class foo
{
   var $mysqli = new mysqli( 'host', 'name', 'pass', 'directory' );
}

Try the Demo to see what does not work here.

So maybe one (!) reason to write

class foo
{
   var $mysqli;
   public function __construct()
   {
       $this->mysqli = new mysqli( 'host', 'name', 'pass', 'directory' );
   }
}

is because there is no alternative to it (between the two alternatives you offered in your question only, naturally. This is horrorful bad practice to do so w/o a delicate design based on dependency injection and other high-level design abstractions - or more precise: This is making mysqli a dependency of foo - including the database configuration).

Next to that, please don't use var, use private, protected or public instead.