查找变量并使用str_replace WP将其替换为变量

I'm trying to use str_replace in my custom plugin to find a variable, replace with and different variable, and search for it in the html5 header tag... like this

if ( is_page( "Event Photo Uploader" )) {

    $revised_subtitle = 'Event Photo Uploader';

    $new_subtitle = str_replace( $subtitle, $revised_subtitle, "<header" );
    return $new_subtitle;

}

This is failling to work for me and I don't know why. I'm still a bit new to this more advanced stuff.

Thanks!