This function deinitializes an encryption module (PHP 4 >= 4.1.1, PHP 5)
bool mcrypt_generic_deinit ( resource td )

This function terminates encryption specified by the encryption descriptor (td). It clears all buffers, but does not close the module. You need to call mcrypt_module_close() yourself. (But PHP does this for you at the end of the script.) Returns FALSE on error, or TRUE on success.

See for an example mcrypt_module_open() and the entry on mcrypt_generic_init().