Call to undefined function imagecreatefrompng()

sudo apt-get install php7.0-gd

PDO mysql extension is not installed

sudo apt-get install php7.0-gd php7.0-mysql

The mbstring extension is missing

sudo apt-get install php-mbstring php7.0-mbstring php-gettext libapache2-mod-php7.0

then

sudo phpenmod mcrypt
sudo phpenmod mbstring
sudo service apache2 restart