& is bitwise AND. See Bitwise Operators. Assuming you do 14 & 7:
14 = 1110
7 = 0111
---------
14 & 7 = 0110 = 6
&& is logical AND. See Logical Operators. Consider this truth table:
$a $b $a && $b
false false false
false true false
true false false
true true true