Call to undefined function mysql_pconnect()

Wow. This took ages.
A customer of mine has developed a PHP application on his windows machine (yeah yeah I know). I tried installing it on my Linux laptop – and all I got was a white page.
Well, loving the challenge, I started putting in debu prints in all the PHP files. Turns out the customer uses CodeIgniter, so I started with core/CodeIgniter.php, moved to core/Loader.php, core/db/DB.php and ended up with the mysql_driver.php.
Turns out I was crashing on this line:

return @mysql_pconnect($this->hostname, $this->username, $this->password);

The error message (after removing the @ of course) was
Call to undefined function mysql_pconnect()
So lame. I forgot to run sudo apt-get install php5-mysql. 2 hours of my life wasted 🙁