为什么var_dump（（int）（PHP_INT_MAX + 1））是负数？

I pulled this result from here:

And interestingly, the result of var_dump((int)(PHP_INT_MAX + 1)) will be displayed as a negative number (in the case of this specific example, it will display int(-9223372036854775808)). Again, the key here is for the candidate to know that the value will be displayed as a negative number.

Is the int negative because adding 1 will overflow the integer bits and change the bit representing the sign of the int? What's the reason?