PHP Compatibility

Hi

I ran the plugin through a php scanner for 7.3 and it reported:

=== Youtube Showcase ===
File: plugins/youtube-showcase/includes/common-functions.php
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$glob_limit}
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$glob_orderby}

File: plugins/youtube-showcase/includes/integration-shortcodes.php
Global with anything other than bare variables is discouraged since PHP 7.0. Found $ {\n\t\t\t\t$shc_filter\n\t\t\t}

File: plugins/youtube-showcase/includes/shortcode-functions.php
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$set_list}
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$shc_count_var}
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$count_var}
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$shc_filter}
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$count_var}
Global with anything other than bare variables is discouraged since PHP 7.0. Found ${$count_var}