Fatal error when you Speed Booster Plugin

When using the Speed Booster plugin.

Anytime I add a code such is this to the td-speed-booster.php file

$this->move_js_to_footer(‘media-upload’);

I am getting this error

Fatal error: Using $this when not in object context in

Maybe I am placing the code in the wrong area.

I am following the Page Speed guide:

https://forum.tagdiv.com/page-speed-guide/

Please advise

Lucian, thanks for replying. I am still getting same error.

By placing the code below in the wp-content/plugins/td-speed-booster/td-speed-booster.php file

$this->move_js_to_footer(‘thickbox’);

I get this error:

Parse error: syntax error, unexpected T_VARIABLE, expecting T_FUNCTION in

I put this code under the line

var $styles_for_footer = array(); // here we keep all the stylesheets IDs that we want to move

Thanks