There is nothing wrong with the code... it's how you have it formatted.
Code:
<?php
$banners = array('<a href="http://www.shareasale.com/m-pr.cfm?merchantID=14282&userID=270588&productID=466753644"><img border="0" src="http://www.fightpassport.com/wp-content/uploads/2009/12/mma-gear-13.jpg "></a>','<a href="http://www.shareasale.com/m-pr.cfm?merchantID=14282&userID=270588&productID=466753674"><img border="0" src="http://www.fightpassport.com/wp-content/uploads/2009/12/mma-gear-11.jpg"></a>','<a href="http://www.shareasale.com/m-pr.cfm?merchantID=14282&userID=270588&productID=466753670"><img border="0" src="http://www.fightpassport.com/wp-content/uploads/2009/12/mma-gear-10.jpg"></a>','<a href="http://www.shareasale.com/m-pr.cfm?merchantID=14282&userID=270588&productID=466753642"><img border="0" src="http://www.fightpassport.com/wp-content/uploads/2009/12/mma-gear-7.jpg"></a>','<a href="http://www.shareasale.com/m-pr.cfm?merchantID=14282&userID=270588&productID=459380515"><img border="0" src="http://www.fightpassport.com/wp-content/uploads/2009/12/mma-gear-6.jpg"></a>','<a href="http://www.shareasale.com/m-pr.cfm?merchantID=14282&userID=270588&productID=467926268"><img border="0" src="http://www.fightpassport.com/wp-content/uploads/2009/12/mma-gear-3.jpg"></a>');
shuffle($banners);
echo $banners[0];
?>