Adds filtername to the list of filters attached to stream. This filter will be added with the specified params to the beginning of the list and will therefore be called first during stream operations. To add a filter to the end of the list, use stream_filter_append().
By default, stream_filter_prepend() will attach the filter to the read filter chain if the file was opened for reading (i.e. File Mode: r, and/or +). The filter will also be attached to the write filter chain if the file was opened for writing (i.e. File Mode: w, a, and/or +). STREAM_FILTER_READ, STREAM_FILTER_WRITE, and/or STREAM_FILTER_ALL can also be passed to the read_write parameter to override this behavior. See stream_filter_append() for an example of using this parameter.
When using custom (user) filters: stream_filter_register() must be called first in order to register the desired user filter to filtername.
See also stream_filter_register(), and stream_filter_append()