pdo_mysql and gd recognised/enabled in terminal but not on phpinfo()

Short answer: the output from php -m is coming from the CLI PHP binary, while phpinfo() shows the PHP instance Apache is running. Those are often different SAPIs with different php.ini files and loaded-extension lists, so seeing GD and pdo_mysql in the terminal but not in phpinfo() is a classic CLI-vs-Apache mismatch. (stackoverflow.com)

Do these checks (they’ll point exactly at which php and which php.ini Apache is using):

which -a php
php -v
php --ini
php -m

# create a web-visible test page (remove it afterwards)
# info.php contents:
<?php phpinfo(); ?>

Open that info.php in a browser and look for “Server API”, “Loaded Configuration File”, and “Additional .ini files parsed”. php --ini / php -i show the CLI config; phpinfo() shows the web config. (php.net)

If Apache’s PHP is missing GD/PDO, enable them in the php.ini that phpinfo() lists (or put small .ini files in the “scan” directory). Typical fixes: ensure extension_dir points to the directory containing the .so files (or add extension=gd.so / extension=pdo_mysql.so lines), save, then restart Apache:

sudo apachectl restart
# then check Apache error log for startup/load errors:
sudo tail -n 200 /var/log/apache2/error_log

If an extension won’t load you’ll often see a dependency or binary-mismatch error in the logs. After a successful restart the extensions should appear in phpinfo(). (php.net)

Extra tips: use which -a php and apachectl -V to find multiple installs and the ServerRoot; make a backup copy of any php.ini before editing; delete the temporary phpinfo page when finished because it exposes server details. (developer.wordpress.org)

(Direct testing of GD as @FlashCreations suggested — a tiny PHP script that creates and outputs an image — is a good final verification once the extension is enabled.)