So I'm re-doing a web app using PDO rather than the built-in MySQL stuff as there seems to be a possibility that the MySQL extension will be gone before too long.
So after looking around I discovered that if I alter my SQL I can check on one line thus:
$dbconn->query("SELECT COUNT(*) FROM `something` WHERE `condition` = 'something'")->fetchColumn()
Which'll give me the number of matching records. I can then check that against a condition and carry on... or not as the case may be.