The & is a 'bitwise and' operator and is used in doing arithmetic on binary. For the example that you gave 'a' is binary 01100001 and 'b' is binary 01100010. The & means to set the result's bits to 1 where it is set in both, and to 0 otherwise. So the result is 01100000. Now, the echo statement converts the result to a string and the ASCII character for 01100000 (decimal number 96) is the backtick (`) character.