learn the basics first, then build on it, dont bite off more than you can chew.
namely:
Variables
Functions
Arrays

also i'd add to the list, data exchange:
XML
RSS
JSON

and maybe even the PHP XSLT stuff too
and .htaccess is really more of an apache thing, but never the less still good to learn

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)