在类构造函数外部分配变量以在类中使用？

I'm just beginning to work with class constructors. In the script below, I want to pass in the $arg2 value from outside the class.

How do I need to define a variable $someVariable=n, so that it can be set from outside the class constructor from the parent file which includes the file below?

class myClassTest
{       
    public $var1;
    public $var2;
    public $var3;

    function __construct($arg1,$arg2=$someVariable){ //MAKE $arg2 dynamically set from outside the class
        $this->var1 = $arg1;
        $this->var2 = $arg2;
        $this->var3 = array();
    }

Just use it like this, however it's not recommended

$someGlobalVar = "test";
class myClassTest
{       
    public $var1;
    public $var2;
    public $var3;

    function __construct($arg1,$arg2=null){
        if ($arg2 === null){
            global $someGlobalVar;
            $arg2 = $someGlobalVar;
        }
        echo $arg2;
        $this->var1 = $arg1;
        $this->var2 = $arg2;
        $this->var3 = array();
    }
 }
 $class = new myClassTest('something'); //outputs test

working demo

You can't set the default value of argument $arg2 using some 'external' variable. Default values get (logically) set at 'definition time' of the function. Thus, these parameters needs to be literal (constant) values.

Thus, these are fine declarations:

   function makecoffee($type = "cappuccino") { }
   function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL) { } 

If you want to 'inject' external stuff, you need to do something like this:

$someglobalVariable = 'whatever';

class myClassTest
{       
    public $var1;
    public $var2;
    public $var3;

    function __construct($arg1,$arg2=null){ //MAKE $numres dynamic from outside the class

        global $someglobalVariable;

        if ( ! isset( $arg2 ) ) {
           $this->var2 = $someglobalVariable;
        } else {
           $this->var2 = $arg2;
        }
        $this->var1 = $arg1;
        $this->var3 = array();
    }

} // end of class

Note, that it is bad style to access global variables in PHP (as in any other object-oriented language).