title: How to keep PHP 5.5 but use 5.4 (CodeIgniter deprecated)
body: <p>I have a dedicated server running with ISPConfig 3.
  It came under Debian 7 (officially PHP 5.4).
  I have upgraded it to PHP 5.5.</p>
  
  <p>However, I didn't realize that CodeIgniter needs the mysql driver to run... Therefore, my website doesn't work anymore.</p>
  
  <p>So either I find a way to use PHP 5.4 only on a website (without affecting the others running under PHP 5.5) or I find a way to "upgrade" CodeIgniter so it doesn't use the deprecated mysql extension anymore.</p>