I do not understand why properties are declared.

<?php
class Student{
 private $name;
 public $age;
 public function __construct($name,$age){
   $this->name=$name;
   $this->age=$age;
   $this->sum=$name+$age;
 }
}
$tmpStudent=new Student("Mindi","22");
echo "Age : ". $tmpStudent->age;
echo "<br>Sum:".$tmpStudent->sum;

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I am typing from a code tutorial.

I think a good way of learning is by adding lines to the existing code.

So I added $this->sum which still displayed but did not need to be declared as a property above the constructor.

Why are properties declared?