I just installed the app for the first time and got the same error as well. Thankfully, this error does not affect the installation. If you only receive this error (along with something about not being able to set the header), the app should have installed fine. All it does at the end of the installation script is forward you to the top-level directory.
The reason this error appears is because in more recent releases of PHP, array_replace() is a built-in function. And since the programmer for FA wrote his own array_replace() function, PHP is just telling you that they conflict.