[PHP-NOTES] note 130443 added to function.filter-var

var_dump(filter_var(null, FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE)); <- returns false

you would expect this to return null because it uses FILTER_NULL_ON_FAILURE 

"On success returns the filtered data. On failure false is returned, unless the FILTER_NULL_ON_FAILURE flag is used, in which case null is returned."

The filter_var() function first coerces null to an empty string before applying the filter so it is actually doing

var_dump(filter_var('', FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE));  <- returns false
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