What programming languages do you already know? If you know any C style languages; C++, Java, etc., the only thing that will be foreign will be PHPs relaxed array handling, no explicit type declarations, a few object handling oddities and a few funky syntax/logical/scoping differences. Apart from that you will just need to learn PHPs infamous wide range of inconsistently named functions, libraries, variables, etc. So pretty much just keep http://www.php.net/manual bookmarked.
This site might be good, http://www.tizag.com/phpT/syntax.php -- It has a lot of content you can skim through on the left and the pages are pretty short to get you rolling quick.
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