Description:
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When using a user created php_user_filter, attached to a url stream (with fopen), the error "Warning: stream_filter_append(): Filter failed to process pre-buffered data in ..." occured. 

- There is/was no error in PHP 5.6.27. 
- There is no error if fopen open a local file. 
- There is no error with the integrated 'string.toupper' filter.

Test script:
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Short :
// Example class strtoupper in http://php.net/manual/en/function.stream-filter-register.php#example-4575
stream_filter_register('strtoupper', 'strtoupper_filter');
$fp = fopen('http://localhost/test.txt', 'r');
stream_filter_append($fp, 'strtoupper', STREAM_FILTER_READ); # DOES NOT WORK

Complete example :
https://gist.github.com/anonymous/7b22c14390d3e831a8d472aad00baeee

Expected result:
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No error (and the text print in uppercase)

Actual result:
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No error in PHP 5.6
Error in PHP 7.0.12 (Warning: stream_filter_append(): Filter failed to process pre-buffered data in xxx)