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When testing the string '0' for validity as 'int', the modifier returns the empty string.

This is because the conditional test for simple equality to TRUE:

modifier.validate.php (Feb 10 2009) :
if (array_key_exists($type, $filters) && filter_var($string, $filters[$type]))

Since filter_var returns the filtered data, in this case 0, this test is FALSE. Instead this line should be like this:

if (array_key_exists($type, $filters) && filter_var($string, $filters[$type]) !== FALSE)