Another use for random numbers is selecting banner ads. Suppose you've signed up three sponsors for your Web site. Each has a single banner you promise to display on an equal proportion of hits to your site. To accomplish this, generate a random number and match each number to a particular banner. In Listing 21.10, I've used a switch statement on a call to mt_rand. In a situation like this, you don't need to worry too much about using good seeds. You simply want a reasonable distribution of the three choices. Someone guessing which banner will display at midnight poses no security risk.
<?php //Seed the generator mt_srand(doubleval(microtime()) * 100000000); //choose banner switch(mt_rand(1,3)) ...