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Simple HTML Dom - Fatal error when using load_file
You need to install the mbstring PHP extension. Your previous shared server must have had it installed, but your dedicated server does not.
Perhaps your web hosting company can install it. If not, you'll have to do it yourself. How it is installed depends on your operating system on the server. But if you Google "install mbstring php extension on your OS here", you should be able to find what you're looking for. If not, then to help any further we'll need to know the operating system and version.