I think you have a bug in your script:
Excuse my English. :-)
If an bad-intentioned user buys the Magic Ball, he can insert any html code in the "question" field. That´s very dangerous, because he can obtain the admin password, or hack the forum.....
Here is the solution:
You have to replace the code from $sizeof = count($answer); to </center>"; with this one:
$sizeof = count($answer);
$random = (rand()%$sizeof);
//make the question a variable
$cambiar = $_POST['question'];
// Change al the < and >
$cambiar = str_replace("<","‹",$cambiar);
$cambiar = str_replace(">","›",$cambiar);
// print all in the page
return "<center><img src='
http://xxxx.lomasinteresante.net/Sources/shop/xx/8ball.jpg' width=300 height=320><br />Tu pregunta fue $cambiar<br /><br />La bola responde:<br /><br />$answer[$random]</center>";
Note: Don´t forget to change the path for your image and translate al to your language.