Description:
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If a string containing an unmatched "<" character is run through the FILTER_SANITIZE_STRING filter the string is truncated at the <.

The problem seems to stem from the last parameter in the call to php_strip_tags_ex(). That parameter tells php_strip_tags_ex() ignore spaces trailing "<" characters. I checked how php_strip_tags_ex() is called in the PHP function strip_tags() and it tells php_strip_tags_ex to allow spaced after a "<".

See ext/filter/santitizing_filters.c line 203 and ext/standard/string.c line 4023 in PHP 5.3.5.

Test script:
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echo filter_var('four is < 6', FILTER_SANITIZE_STRING);

Expected result:
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four is < 6

Actual result:
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four is