stream_filter_register allows you to implement your own filter on any registered stream used with all the other filesystem functions (such as fopen, fread etc.).
The filter name to be registered.
To implement a filter, you need to define a class as an extension of php_user_filter with a number of member functions. When performing read/write operations on the stream to which your filter is attached, PHP will pass the data through your filter (and any other filters attached to that stream) so that the data may be modified as desired. You must implement the methods exactly as described in php_user_filter - doing otherwise will lead to undefined behaviour.
Returns true on success or false on failure.
stream_filter_register will return false if the filter_name is already defined.