PHP stream_filter_prepend() Function

The PHP stream_filter_prepend() function adds a filter to the list of filters attached to the given stream. This function adds the specified filter to the beginning of the list and will therefore be called first during stream operations.

To add the filter to the end of the list, stream_filter_append() function can be used.

Syntax

stream_filter_prepend(stream, filtername, read_write, params)

Parameters

`stream`	`Required.` Specify the target stream.
`filtername`	`Required.` Specify the filter name.
`read_write`	`Required.` By default, this function attaches 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.
`params`	`Optional.` This filter will be added with the specified params to the beginning of the list and will therefore be called first during stream operations.

Return Value

Returns a resource on success or false on failure. Returns false if stream is not a resource or if filtername cannot be located.

Example: controlling where filters are applied

The example below shows the usage of stream_filter_prepend() function.

<?php
//open a test file for writing
$fp = fopen("test.txt", "w");

//prepending 'string.rot13' filter to the file stream 
$rot13_filter = stream_filter_prepend($fp, "string.rot13", 
                                      STREAM_FILTER_WRITE);

//writing some content with filter attached                                
fwrite($fp, "This is ");

//removing the $rot13_filter filter
stream_filter_remove($rot13_filter);

//writing some content after removing filter
fwrite($fp, "a test\n");

//setting the file pointer to the start
rewind($fp);

//closing the file
fclose($fp);

//reading and displaying the content of the file
echo file_get_contents("test.txt");
?>

The output of the above code will be:

Guvf vf a test

Note: Stream data is read from resources (both local and remote) in chunks, with any unconsumed data kept in internal buffers. When a new filter is prepended to a stream, data in the internal buffers, which has already been processed through other filters will not be reprocessed through the new filter at that time. This differs from the behavior of stream_filter_append().

Note: When a filter is added for read and write, two instances of the filter are created. This function must be called twice with STREAM_FILTER_READ and STREAM_FILTER_WRITE to get both filter resources.

❮ PHP Streams Reference