PHP is evil.
PHP seems to have frequent version updates. Thanks, Zend, et al. PHP is now on version 5 and PHP folks are looking forward to 6. But my server and most of my clients' servers are still on PHP 4.3.1. And just like the jump from Flash Actionscript 1.0 to 2.0, there is a big difference between PHP 4 and PHP 5, what fucnctions and coded that will work in PHP 5 are not necessarily backwards compatible.
All my current reference books are for PHP 5. Very delicious ideas and scripts but not so delicious trying to implement them on servers with no immediate intentions on upgrading to PHP 5. My server allows one to append PHP 5 in the .htaccess file and call it in the file extension, but this is not a reliable fix. Add security on top of it... Grrr... Grrr... Grr...
Must improve my Perl fu.