Can you'all educate me about these mythical rounding errors that require using GMP?
I can see, maybe, if you're computing interest down to the penny on a 30-year mortgage you might conceivably be off by a penny if you use 64-bit floats instead of 64-bit integers, although even there you're going to have to think hard about rounding as you get integer remainders.
And I can see being really careful if you're writing a bitcoin exchange site or bitcoin bank that deals in thousands of internal transactions that must all balance exactly.
But for the typical PHP website that is just going to add up 10 items in a shopping cart using plain-old PHP Numbers will be just fine. I don't see PayPal recommending that PHP users of it's APIs install GMP. Recommending that any website dealing with bitcoins compile genjix' fork and use GMP is a really good way to ensure that nobody accepts bitcoins.