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+################
+Common Functions
+################
+
+CodeIgniter uses a few functions for its operation that are globally
+defined, and are available to you at any point. These do not require
+loading any libraries or helpers.
+
+is_php('version_number')
+==========================
+
+is_php() determines of the PHP version being used is greater than the
+supplied version_number.
+
+::
+
+	if (is_php('5.3.0')) {     $str = quoted_printable_encode($str); }
+
+Returns boolean TRUE if the installed version of PHP is equal to or
+greater than the supplied version number. Returns FALSE if the installed
+version of PHP is lower than the supplied version number.
+
+is_really_writable('path/to/file')
+====================================
+
+is_writable() returns TRUE on Windows servers when you really can't
+write to the file as the OS reports to PHP as FALSE only if the
+read-only attribute is marked. This function determines if a file is
+actually writable by attempting to write to it first. Generally only
+recommended on platforms where this information may be unreliable.
+
+::
+
+	if (is_really_writable('file.txt')) {     echo "I could write to this if I wanted to"; } else {     echo "File is not writable"; }
+
+config_item('item_key')
+=========================
+
+The :doc:`Config library <../libraries/config>` is the preferred way of
+accessing configuration information, however config_item() can be used
+to retrieve single keys. See Config library documentation for more
+information.
+
+show_error('message'), show_404('page'), log_message('level',
+'message')
+==========
+
+These are each outlined on the :doc:`Error Handling <errors>` page.
+
+set_status_header(code, 'text');
+==================================
+
+Permits you to manually set a server status header. Example::
+
+	set_status_header(401); // Sets the header as:  Unauthorized
+
+`See here <http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html>`_ for
+a full list of headers.
+
+remove_invisible_characters($str)
+===================================
+
+This function prevents inserting null characters between ascii
+characters, like Java\\0script.
+
+html_escape($mixed)
+====================
+
+This function provides short cut for htmlspecialchars() function. It
+accepts string and array. To prevent Cross Site Scripting (XSS), it is
+very useful.