(Google website optimizer kind of) randomization in PHP (but smart, quick and slim)

Every time someone visits my site, I show one of three options (A, B, C). If the user likes the option, he clicks on it. I want to find a way to show the options that receive less clicks less frequently. What's the best way of doing this in PHP?

I am saving the clicks in MongoDB by simply adding a "vote" in an array:

$option[]='a';//one click on option A
$option[]='b';//one click on option B
$option[]='b';//another click on option B

try{
 $m=new Mongo();
 $c=$m->db->clicks;
 $c->save($option);
 $m->close();
}
catch(MongoConnectionException $e){ die('Error connecting to MongoDB server. ');}
catch(MongoException $e){ die('Error: '.$e->getMessage());} 

This prints:

Array
(
    [0] => a
    [1] => b
    [2] => b
)

Not sure if I understand your question correctly. But if I did, then the following is perhaps a rather naive, and perhaps even verbose way of doing what I think you want to do:

// assume the following fictional values,
// that is, the amount of clicks each option has received thusfar
$clicks = array(
  'A' => 10,
  'B' => 40,
  'C' => 50
);

// what is the total amount of clicks?
$totalClicks = array_sum( $clicks );

// determine the lower bound percentages of the option clicks
$clickPercentageBounds = array(
  'A' => 0,
  'B' => ( ( $clicks[ 'A' ] + 1 ) / $totalClicks ) * 100,
  'C' => ( ( $clicks[ 'A' ] + $clicks[ 'B' ] + 1 ) / $totalClicks ) * 100
);

// get random percentage
$rand = mt_rand( 0, 100 );

// determine what option to show, based on the percentage
$option = '';
switch( true )
{
    case $rand < $clickPercentageBounds[ 'B' ]:
        $option = 'A';
        break;
    case $rand < $clickPercentageBounds[ 'C' ]:
        $option = 'B';
        break;
    default:
        $option = 'C';
        break;
}

var_dump( $option );