no, arithmetic.
if it were coding, it'd be peasy..

echo $num[floor(strlen($num)/2)];

( Jenk Neptune is a pedant and now I can't fit my sig in, Wed 17 Jun 2009, 18:02, archived)

All decimal numbers can be written "long-hand" as
c₀ × 10⁰ + c₁ × 10¹ + c₂ × 10² + ... + c_n × 10ⁿ (more typically written as c_n...c₂c₁c₀; e.g. 439 is 4 × 10² + 3 × 10¹ + 9 × 10⁰.)

you should first find n, divide it by two, and you've got the power of 10 the middle digit multiplies. From there, it gets a little complicated.
( The Mock TurtIe ™ --- Thinks you are a cunt, on, Wed 17 Jun 2009, 18:16, archived)