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. Thatīs 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: Donīt forget to change the path for your image and translate al to your language.