callback function return return($var & 1)?

Question

I have read the PHP Manuel about array_filter

Answer 1

& 

it's the bitwise operator. It does the AND with the corrispondent bit of $var and 1

Basically it test the last bit of $var to see if the number is even or odd

Example with $var binary being 000110 and 1

000110 &
     1
------
     0

0 (false) in this case is returned so the number is even, and your function returns false accordingly