Historically, the reason for this discrepancy is that internal functions have supported a concept of scalar types (bool, int, float, string) long before they were introduced for user-defined functions in PHP 7.0, and the existing implementation silently accepted null values. For the new scalar type declarations introduced in PHP 7.0 an explicit choice was made to not accept null values to non-nullable arguments, but changing the existing behavior of internal functions would have been too disruptive at the time.