PHP Version 5.6 Formula was removed
Last versions were

Revision: 9

php56 requires the following formulae to be installed:
enchant 2.2.3 Spellchecker wrapping library
freetype 2.9 Software library to render fonts
gettext GNU internationalization (i18n) and localization (l10n) library
gmp 6.1.2_2 GNU multiple precision arithmetic library
icu4c 61.1 C/C++ and Java libraries for Unicode and globalization
jpeg 9c Image manipulation library
libvpx 1.7.0 VP8/VP9 video codec
libpng 1.6.34 Library for manipulating PNG images
libxml2 2.9.7 GNOME XML library
unixodbc 2.3.6 ODBC 3 connectivity for UNIX
readline 7.0.3_1 Library for command-line editing
openssl 1.0.2o_1 SSL/TLS cryptography library
httpd 2.4.33 Apache HTTP server

Recent formula history

ilovezfs php56: migrate to homebrew/core (#4835)
Sean Molenaar php56: 5.6.33
Andreas Braun php56 5.6.32
Andreas Braun php56 5.6.31
Jan Viljanen php56: 5.6.30

Formula code at GitHub