$str = 'RM,RMVB,MKV,MP4,MPEG,MPG,DAT,flv,m3u8,wmv,avi,3gp,amv,dmv';
//转换为小写
$str = strtolower($str);
$ary = explode(',',$str);

//回调函数 修改每个值元素
print_r(array_reduce($ary,'myfunction',''));
function myfunction($v1,$v2) 
{
	return $v1 . "'" . $v2."',";
}
exit();

也可以用map很方便

$str = 'RM,$str);

print_r(array_map("myfunction",$ary));
function myfunction($v1) 
{
	return "*." . $v1."; ";
}
exit;