Where can I get the proper srpms that contains php-imp?
Like Fabian said, it's in php-extras, part of epel: http://dl.fedoraproject.org/pub/epel/7/SRPMS/p/php-extras-5.4.16-7.el7.src.r...
Where can if find complete instructions on building from said srpms?
I would use mock, in stead of rpmbuild. mock will create a build environment, so that builds are repeatable.
man page: https://linux.die.net/man/1/mock
a fairly long overview: https://fedoraproject.org/wiki/How_to_create_an_RPM_package
Jacco