This parameter determines what lexer implementation can be used. The
valid values are:
null
Recommended, the lexer implementation will be auto-detected based on
your PHP-version and configuration.
string lexer identifier
This is a slim way of manually overridding the implementation.
Currently recognized values are: DOMLex (the default PHP5
implementation)
and DirectLex (the default PHP4 implementation). Only use this if
you know what you are doing: usually, the auto-detection will
manage things for cases you aren't even aware of.
object lexer instance
Super-advanced: you can specify your own, custom, implementation that
implements the interface defined by HTMLPurifier_Lexer.
I may remove this option simply because I don't expect anyone
to use it.