callback function return return($var & 1)?

Question

I have read the PHP Manuel about array_filter

Answer 1

$var & 1 - is bitwise AND it checks if $var is ODD value

0 & 0 = 0,
0 & 1 = 0,
1 & 0 = 0,
1 & 1 = 1 

so, first callback function returns TRUE only if $var is ODD, and second - vise versa (! - is logical NOT).