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constants may not be redefined or undefined once they have been set; and
el tercer parámetro opcional case_insensitive también se encuentra disponible.

all registered variables are serialized after the request finishes. registered variables which are undefined are marked as being not defined. on subsequent accesses, these are not defined by the session module unless the user defines them later.
las opciones de configuración track_vars y register_globals influyen notablemente en la forma en que las variables de la sesión se almacenan y restauran.

call_user_function() returns success on success, and failure in case the function cannot be found. you should check that return value! if it returns success, you are responsible for destroying the retval pval yourself (or return it as the return value of your function). if it returns failure, the value of retval is undefined, and you mustn't touch it.
si devuelve success, debe usted ocuparse de destruir el pval devuelto (o devolverlo como el valor de retorno de su función).