I have actually built a desktop application that can send bulk SMS so I believe I understand exactly how I could do this. The only thing I would have to do is take my Python code and rewrite it in PHP, which I am proficient enough in to finish this task quickly enough. The basic idea is that you use the PHP "sendmail" function to send an SMS message through an email, since every phone number has an associated email, it really is an easy task, the harder part would be setting up the credit system and accepting payments, however this could be done easy enough with some MySQL, PHP and probably the PayPal API. PayPal is probably the most used but integrating more payment options really isnt a big deal. I hope to work with you soon!