Patch using method below or if you don't want to patch a core file, see this Magento extension that does the same thing but correctly overrides the core file:
https://github.com/MageMechanic/PhpSeven
# run in magento root directory
# one-liner to apply patch using wget
wget -qO- 'https://gist.githubusercontent.com/MageMechanic/66449504110fbbd96214/raw/php7_magento_1_9_core.patch' | patch -p0
# one-liner to apply patch using curl
curl -s 'https://gist.githubusercontent.com/MageMechanic/66449504110fbbd96214/raw/php7_magento_1_9_core.patch' | patch -p0
# ...easier to read
PATCH_URL='https://gist.githubusercontent.com/MageMechanic/66449504110fbbd96214/raw/php7_magento_1_9_core.patch'
# to apply with wget
wget -qO- $PATCH_URL | patch -p0
# ...or with curl
curl -s $PATCH_URL | patch -p0
# to revert using wget
wget -qO- $PATCH_URL | patch -p0 -R
# ...or revert using curl
curl -s $PATCH_URL | patch -p0 -R
http://afilina.com/magento-1-9-on-php7/
http://www.bubblecode.net/en/2015/07/24/magento-php-7/
http://magento.stackexchange.com/questions/74008/is-magento-ready-for-php-7#answer-80912
http://www.code007.ro/making-work-magento-with-php-7-rc1/
Is there a Magento2 PR for this fix yet?