Description:
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According to http://php.net/manual/en/filter.filters.flags.php

FILTER_NULL_ON_FAILURE is described as "Returns NULL for unrecognized boolean values.", used with FILTER_VALIDATE_BOOLEAN.

But this flag also affects other, non-boolean, filters.

Test script:
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var_dump(filter_var("123a", FILTER_VALIDATE_EMAIL, FILTER_NULL_ON_FAILURE)); // null
var_dump(filter_var("123a", FILTER_VALIDATE_EMAIL)); // false

var_dump(filter_var("123a", FILTER_VALIDATE_INT, FILTER_NULL_ON_FAILURE)); // null
var_dump(filter_var("123a", FILTER_VALIDATE_INT)); //false

var_dump(filter_var("123a", FILTER_VALIDATE_IP, FILTER_NULL_ON_FAILURE)); // null
var_dump(filter_var("123a", FILTER_VALIDATE_IP)); //false

// etc