PHP Classes: when to use :: vs. ->?

Question

I understand that there are two ways to access a PHP class - \"::\" and \"->\". Sometime one seems to work for me, while the other doesn\'t, and I don\'t understand why.

Answer 1

:: is used to access a class static property. And -> is used to access a class instance ( Object's ) property.

Consider this Product class that has two functions for retrieving product details. One function getProductDetails belongs to the instance of a class, while the other getProductDetailsStatic belongs to the class only.

class Product {
  protected $product_id;

  public function __construct($product_id) {
    $this->product_id = $product_id;
  }

  public function getProductDetails() {
     $sql = "select * from products where product_id= $this->product_id ";
     return Database::execute($sql);
  }

  public static function getProductDetailsStatic($product_id) {
     $sql = "select * from products where product_id= $product_id ";
     return Database::execute($sql);
  }
}

Let's Get Products:

$product = new Product('129033'); // passing product id to constructor
var_dump( $product->getProductDetails() ); // would get me product details

var_dump( Product::getProductDetailsStatic('129033') ); // would also get me product details

When to you use Static properties?

Consider this class that may not require a instantiation:

class Helper {
  static function bin2hex($string = '') {
  }
  static function encryptData($data = '') {
  }
  static function string2Url($string = '') {
  }
  static function generateRandomString() {
  }
}