Template administration Error after WP 4.8 update

Question

We are using UpStream template on our WP site. After the last WP 4.8 update, we are getting an error message on the administration panel, which does not allow us to use the slideshow. The error is the following:

Fatal error: Uncaught Error: Function name must be a string in /var/www/vhost/calamburteatro.com/home/html/wp-content/themes/UpStream/lib/Admin.php:248 Stack trace: #0 /var/www/vhost/calamburteatro.com/home/html/wp-content/themes/UpStream/lib/Admin.php(451): ThematerAdmin->form_callback('featuredposts_s...', Array) #1 /var/www/vhost/calamburteatro.com/home/html/wp-content/themes/UpStream/lib/Admin.php(388): ThematerAdmin->optionsPageContentItems(Array) #2 /var/www/vhost/calamburteatro.com/home/html/wp-content/themes/UpStream/lib/Admin.php(324): ThematerAdmin->optionsPageContent() #3 /var/www/vhost/calamburteatro.com/home/html/wp-includes/class-wp-hook.php(298): ThematerAdmin->ThematerThemeOptions('') #4 /var/www/vhost/calamburteatro.com/home/html/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array) #5 /var/www/vhost/calamburteatro.com/home/html/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #6 /var/www/vhost/calamburteatro.com/home/html/wp-admin/admin.php(222): do_action('appearance_page...') #7 / in /var/www/vhost/calamburteatro.com/home/html/wp-content/themes/UpStream/lib/Admin.php on line 248

The line 248 on the admin.php is: $callback[0]->$callback[1]();

And the whole fuction code is:

function form_callback ($name, $attributes = array())
    {
        if(isset($attributes['callback']) && is_array(($attributes['callback']))) {
            $callback = $attributes['callback'];
            $callback[0]->$callback[1]();
        } else {
            if(function_exists($name)) {
                return $name($attributes);
            }
        }

    }

Would anybody help us with this, please? Thanks in advance!

Answer 1

This is a generic PHP question, but simple to answer. The problem is most likely caused by PHP 7.

Simply change the line:

$callback[0]->$callback[1]();

to

$callback[0]->{$callback[1]}();

This is because $callback[0]->$callback[1](); means $callback[0]->{$callback[1]}(); in PHP5, while it means ($callback[0]->$callback)[1](); in PHP7.

Take a look into this page to know the details about the change.