Hi, Gabrielle !
This summer I have a deal with Miva Merchant, the system based on Miva script, that intend for fast e-storefront creation.
I have written custom UI module with original interface and incorporate it with Miva Merchant. It's a long story to tell about "PHP vs. MIVA" problem in this forum.
Miva Merchant has a backend part with comprehensive facilities that propose a programmer a fast way to create storefront.
But Miva script itself is not good choice,
in my humble opinion. PHP more powerful and faster scripting tool than Miva script, moreover Miva hasn't interface to SQL-servers realized on UNIX systems as PHP has, only DBaseIV. On Windows NT it can connect to ODBC sources but I didn't try to implement it.
All the more, you have a recent experience in PHP e-commerce development. If I were in your shoes I will choose PHP. But it's only my opinion.
Good luck.
Sincerely,
Andrei