How do I use PHP to convert a single character into a string of characters?

I want to use PHP to find any / that is in a variable which can be called via $_GET. This variable is the end part of a document URL. The URL will be fed into Google Document Viewer. However, the embedded document URL should use %2F instead of /. Is it possible to use PHP to say "every time I see /, I replace it with %2F"?

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Url Encoding:
http://php.net/manual/en/function.urlencode.php

Url Decoding:
http://php.net/manual/en/function.urldecode.php

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