在类构造函数外部分配变量以在类中使用？

Question

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();
    }

Answer 1

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