Fact is :
To specify a boolean literal, use the keywords TRUE or FALSE. Both are case-insensitive.
We assume in moodle:
http://docs.moodle.org/en/Development:Coding_style#Booleans_and_the_null_Value
Booleans and the null Value
Use lower case for true, false and null.
We assume in CodeIgniter:
http://codeigniter.com/user_guide/general/styleguide.html#true_false_and_null
TRUE, FALSE, and NULL keywords should always be fully uppercase.