models.php

<?php

abstract class Model {
    static abstract function getFields();

    static function getFieldsForSQL() {
        return implode(', ', self::getFields());
    }

    static function getAll() {
        return 'SELECT ' . self::getFieldsForSQL() . ' FROM ...';
    }
}


class User extends Model {
    static function getFields() {
        return array('username', 'password');
    }
}


print User::getAll();


// Error is:
// Fatal error: Cannot call abstract method Model::getFields() in ...models.php on line 7