Static class example

static.php

/**
 * Static calculator class.
 *
 * There's no way to dependency inject this for 
 * mocking it out while testing.
 */
class Calculator {
    private function __construct() {}

    public static function add($a, $b) {
        return $a + $b;
    }

    public static function multiply($a, $b) {
        return $a * $b;
    }
}


/**
 * Instance calculator class.
 */
class Calculator {
    public function add($a, $b) {
        return $a + $b;
    }

    public function multiply($a, $b) {
        return $a * $b;
    }
}