"Call to undefined method" - but I know it exists

Question

This line of code:

echo "<strong> {$this->author->last} {$this->date->shortYear()}</strong> ...";

gives me this error:

Fatal error: Call to undefined method Date::shortYear() in /f5/debate/public/libs/Card.php  on line 22

Even though in Date.php (which is included in Card.php):

class Date {
    public $day;
    public $month;
    public $year;

    public function shortYear() {
        return substr($this->year, -2);
    }
}

Answer 1

You're instantiating the wrong Date class. You can use PHP's get_class_methods() function to confirm which methods are available.

Answer 2

If you work in an setup where you automatically upload your changes from your IDE to the web server your checking your pages on. It might accidentally be the case that it didn't upload the file.