<!DOCTYPE section [
            <!ENTITY pensum1 "Nærværende lektionsnoter.">
            <!ENTITY pensum1e "These notes.">
            <!ENTITY php "PHP">
            <!ENTITY rdbms "RDBMS">
            <!ENTITY sql "SQL">
            <!ENTITY tcpip "TCP/IP">
]>

Usage:

...
    <para>This is the way to use &sql; in &php; re these &pensum1e;</para>

Resulting in:

This is the way to use SQL in PHP re these These notes.