Description:
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While the FILTER_FLAG_STRIP_LOW and FILTER_FLAG_STRIP_HIGH flags used 
with FILTER_SANITIZE_STRING work fine, They are just a bit blunt to be 
useful in practice. A more normal application would be to remove any 
'funny' characters from plain text, so for example to allow chars 9, 10, 
13, 32-126. This could be called FILTER_FLAG_PLAIN_TEXT. While this 
could be done using regex, it's probably the most common use case, so 
deserves a dedicated filter flag. This could be flipped around and be 
called FILTER_FLAG_ALLOW_WHITESPACE, overriding STRIP_LOW to allow chars 
9, 10, 13 through.

Similarly, a very common need is to normalize line breaks to \n, \r\n or 
\r.