PHP extension loading in CLI but not in CGI/FastCGI mode

PHP extension loading in CLI but not in CGI/FastCGI mode

Compiled PHP 5.5.1 on Ubuntu 12.04
# php -v
PHP 5.5.1 (cli) (built: Aug 22 2013 02:57:03)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2013 Zend Technologies
PDO MySQL extension loads on CLI:
# php --ri pdo_mysql
pdo_mysql
PDO Driver for MySQL => enabled
Client API version => mysqlnd 5.0.11-dev - 20120503 - $Id:
40933630edef551dfaca71298a83fad8d03d62d4 $
Directive => Local Value => Master Value
pdo_mysql.default_socket => /tmp/mysql.sock => /tmp/mysql.sock
When I access a page using CGI/FastCGI, the PDO MySQL extension does not load
<?php phpinfo(); ?>
PDO
PDO support enabled
PDO drivers sqlite
In both cases, the same extension_dir and php.ini file are being used.
php.ini path: /usr/local/lib/php.ini
extension_dir: /usr/local/lib/php/extensions/no-debug-non-zts-20121212
Contents of extension_dir:
#ls /usr/local/lib/php/extensions/no-debug-non-zts-20121212
mcrypt.so opcache.a opcache.so pdo.so pdo_mysql.so
How can I get PHP running in FastCGI to recognise and load the PDO MySQL
extension?