This is one of the common error messages that seems to appear after installing PHP – in particular under Ubuntu or Debian (where I experienced it). The reason for this is that the PDO version you’ve just installed is too old for the constant to exist, usually because you where naive enough to install the extension from PECL instead of using the default supplied by ubuntu. If you did the same as me:
pecl install pdo pecl install pdo_mysql /etc/init.d/apache2 restart
And everything seems to work, except for that missing constant. What the fsck?!
The reason is that the PECL version of PDO is no longer maintained (I’d suggest to automagically push the newest version to PECL too, just so pecl install and pecl upgrade works as expected). The fact is that when you did pecl install pdo, you destroyed the more recent version provided by the default php5-common package (under Ubuntu, at least).
To solve the problem (and if you also did pecl install pdo_mysql, you’ll have to replace that one too…)
apt-get install --reinstall php5-common php5-mysql php5-mysqli
Restart. Relive. Re.. eh .. yeah. Rerere.
If you’re building from source, you’ll need to add:
--enable-pdo --with-pdo-mysql --with-pdo-pgsql (and any other driver you need)