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When you encounter the error for uninstallable packages for libmysqlclient18, do the following in terminal:

sudo apt-get install mysql-server --fix-missing
sudo apt-get install mysql-server php5-mysql

If this doesn't fix the error follow these instructions and it will work:

https://www.raspberrypi.org/forums/viewtopic.php?t=191380#p1202123

sudo rm -r /var/lib/apt/lists

sudo mkdir -p /var/lib/apt/lists/partial

sudo apt-get clean

sudo apt-get update

sudo apt-key list | grep expired

Get the key to use from that result:
pub 4096R/BE1DB1F1 2011-03-29 [expired: 2014-03-28]
sudo apt-key adv --recv-keys --keyserver keys.gnupg.net BE1DB1F1

sudo apt-get update