Using variables in localization strings
From MoodleDocs
Language strings are defined as follows:
$string['stringname'] = 'This is a string';
and are called via:
get_string("stringname","wherefrom")
If you wish to use a variable from the calling page in the string then you will need to use the following variation to call it:
get_string("stringname","wherefrom","passedvariable")
The variable can then be used in your string as follows:
$string['stringname'] = 'This is a string with a variable called $a';
Moodle 2.0
Note the use of { } in Moodle 2.0 language files.
$string['stringname'] = 'This is a string with a variable called {$a}';