As far as I can tell, this package doesn't need PEAR at all.
commit 4cb2d935fc113150329d82ad1c1ca9ffb7d0a4b6 Author: Michael Orlitzky <mjo@gentoo.org> Date: Sat Apr 8 14:44:50 2017 -0400 dev-php/PEAR-Math_BigInteger: new revision sans PEAR dependency. Gentoo-Bug: 614892 Package-Manager: Portage-2.3.3, Repoman-2.3.1