callback function return return($var & 1)?

Question

I have read the PHP Manuel about array_filter

Answer 1

& is a bitwise AND on $var.

If $var is a decimal 4, it's a binary 100. 100 & 1 is 100, because the right most digit is a 0 in $var - and 0 & 1 is 0, thus, 4 is even.