Adding External SMTP server details to existing scripts

Hello All I had a few scripts made a few years ago for me in order to send out invoices to my clients and I want to use an external SMTP server on this scripts but as they are custom made right now they only use the servers internal SMTP server. So what I need is the scripts to be customised for me to be able to enter the details so it shoots email out via the external server. The first script I have is a smaller script and just sends out an invoice then I click a button, the other script I have attaches pdf's to emails as they are sent, sends out reminders, confirmation emails etc. Scipts are designed in php using mysql. Hope someone can please help, thank you very much.