How do I convert Word smart quotes and em dashes in a string?

I have a form with a textarea. Users enter a block of text which is stored in a database.

Occasionally a user will paste text from Word containing smart quotes or emdashes. Those characters appear in the database as: –, ’, “ ,â€

What function should I call on the input string to convert smart quotes to regular quotes and emdashes to regular dashes?

I am working in PHP.

Update: Thanks for all of the great responses so far. The page on Joel's site about encodings is very informative: http://www.joelonsoftware.com/articles/Unicode.html

Some notes on my environment:

The MySQL database is using UTF-8 encoding. Likewise, the HTML pages that display the content are using UTF-8 (Update:) by explicitly setting the meta content-type.

On those pages the smart quotes and emdashes appear as a diamond with question mark.

Solution:

Thanks again for the responses. The solution was twofold:

1. Make sure the database and HTML files were explicitly set to use UTF-8 encoding.
2. Use htmlspecialchars() instead of htmlentities().

This sounds like a Unicode issue. Joel Spolsky has a good jumping off point on the topic: http://www.joelonsoftware.com/articles/Unicode.html