PHP's a server side language, so any PHP you write will either be (a) left as-is or (b) processed. If your newsletter contains PHP code to be executed dynamically by the recipient, it's likely to not work. In that case, direct them to a webpage you know works.
(You could use Javascript, too, but many users have it turned off and don't like turning it on.)
Using server side scripts it's not the right way to send eNewsletters, expacially if the number of recipients is over than 10.000. You will have deliverability problems or bandwidth converns (depends on which smtp server you use).
:!: Don't make my mistakes, use an outsourced service if you are looking for a professional solution. I suggest
www.mailupnet.com which does not have a pay per message or pay per contact pricing model, but you pay only a flat monthly fee and you can send whatever you like.
Victor
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