public function testConcat() { $str1 = new Str("Hello, "); $str2 = new Str("world."); $this->assertTrue($str1->concat($str2)->equals("Hello, world.")); $digits = new Str(); foreach (range(0, 9) as $n) { $digits = $digits->concat($n); } $this->assertTrue($digits->equals("0123456789")); }