$number & 1;

http://www.tiaon.com/wordpress/2007/03/27/the-quicker-way-to/

In PHP & or AND is a bitwise operator. When used it performs bitwise operation between two numbers. In the following example, the operator will test if the last bit of $number is 1. Thus successfully tests if $number is even or odd.

Test if a number is odd or even using bitwise operator

PHP