I am trying to get mysqli set up for php5 but apt tries to substitute
php5-mysqli, which doesn't exist. Does the mysql package also carry the mysqli extension, or do I need to find the source and compile it somewhere?
From the description of the package
php5-mysql(version 5.3.10-1ubuntu3.2) :
This package provides modules for MySQL database connections directly from PHP scripts. It includes the generic "mysql" module which can be used to connect to all versions of MySQL, an improved "mysqli" module for MySQL version 4.1 or later, and the pdo_mysql module for use with the PHP Data Object extension.
So, the package
php5-mysql does contain support for MYSQLi extension.