You could use authoroize.net they have a php api that's easy to use or you could use paypal they also have a php api they both have a sandbox (developer account) you can use to test your script
There are so many option you have to choose right and secure payment gateway like merchantinc.com to make payment online. I am using this payment gateway since long time and is faster as well as merchant friendly payment gateway that offers best rates in the industry for credit card processing.
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