You should always pick the latest technology. PHP 5 has many features that PHP 4 does not have. And the Beginning PHP 4 book is incredibly outdated, it was written using an early version of PHP 4. There were many changes to PHP that ensued after the book was released, effectively making most of its examples useless without some hacking. So you should go with the latest version of the langauge. If you find you are after all interested in a PHP 4 book too, pick up Beginning PHP, Apache, MySQL Web Development instead of Beginning PHP 4.
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