// Dependencies
$ apt install build-essential libprotobuf-dev libboost-dev openssl protobuf-compiler

// PHP with the desired extensions; php7.2-dev is required to compile
$ apt install php7.2-cli php7.2-dev php7.2-mysql php7.2-pdo php7.2-xml

// Compile the extension
$ pecl install mysql_xdevapi

// Create its own ini file
$ echo "extension=mysql_xdevapi.so" > /etc/php/7.2/mods-available/mysql_xdevapi.ini

// Use the 'phpenmod' command (note: it's Debian/Ubuntu specific)
$ phpenmod -v 7.2 -s ALL mysql_xdevapi

// A 'phpenmod' alternative is to manually symlink it
// $ ln -s /etc/php/7.2/mods-available/mysql_xdevapi.ini /etc/php/7.2/cli/conf.d/20-mysql_xdevapi.ini

// Let's see which MySQL extensions are enabled now
$ php -m |grep mysql

mysql_xdevapi
mysqli
mysqlnd
pdo_mysql