I got a nasty little shock when deprecated warnings started popping up everywhere on an old WordPress setup, so it was back to Google-the-shit-out-of-the-error-message mode.
The error messages looked like these:
PHP Deprecated: Assigning the return value of new by reference is deprecated in …
Fix: (via Otto on this thread)
This happens because debug messages are being displayed when PHP5 is running PHP4 code. Disable the debug at the beginning of the affected files by adding
error_reporting(0); after the initial parentheses.
The wp-settings.php file has an option that has to be disabled as well.
error_reporting(E_ALL ^ E_NOTICE ^ E_USER_NOTICE);
Warning: Cannot modify header information – headers already sent by …
Fix: (via Samuel B on this thread)
This happens from trailing or leading spaces at the beginning/end of the affected files – remove, save and you should be good.