The following is a list of a few built-in stream filters for use with stream_filter_append(). Your version of PHP may have more filters (or fewer) than those listed here.
It is worth noting a slight asymmetry between stream_filter_append() and stream_filter_prepend(). Every PHP stream contains a small read buffer where it stores blocks of data retrieved from the filesystem or other resource in order to process data in the most efficient manner. As soon as data is pulled from the resource into the stream's internal buffer, it is immediately processed through any attached filters whether the PHP application is actually ready for the data or not. If data is sitting in the read buffer when a filter is appended, this data will be immediately processed through that filter making the fact that it was sitting in the buffer seem transparent. However, if data is sitting in the read buffer when a filter is prepended, this data will NOT be processed through that filter. It will instead wait until the next block of data is retrieved from the resource.
For a list of filters installed in your version of PHP use stream_get_filters().
Each of these filters does precisely what their name implies and correspond to the behavior of a built-in php string handling function. For more information on a given filter, refer to the manual page for the corresponding function.
string.rot13 (since PHP 4.3.0) Use of this filter is equivalent to processing all stream data through the str_rot13() function.
string.toupper (since PHP 5.0.0) Use of this filter is equivalent to processing all stream data through the strtoupper() function.
string.tolower (since PHP 5.0.0) Use of this filter is equivalent to processing all stream data through the strtolower() function.
string.strip_tags (since PHP 5.0.0) Use of this filter is equivalent to processing all stream data through the strip_tags() function. It accepts parameters in one of two forms: Either as a string containing a list of tags similar to the second parameter of the strip_tags() function, or as an array of tag names.
Example M-4. string.strip_tags