A Web based application (whether written in PHP or Perl, or a Java Servlet) runs as a series of batchlets than need to be linked together. PHP provides a series of functions to handle sessions, with the session being maintained via a cookie to provide the key to most of the data, which is retained on the server in either plain files of in an SQL database.

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More about sessions and cookies

Associated topic - PHP - Further Web Page and Network Handling

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