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  <div class="section" id="upgrading-from-3-x-to-4-x">
<h1>Upgrading from 3.x to 4.x<a class="headerlink" href="#upgrading-from-3-x-to-4-x" title="Permalink to this headline">¶</a></h1>
<p>CodeIgniter 4 is a rewrite of the framework and is not backwards compatible.
It is more appropriate to think of converting your app, rather than upgrading it.
Once you have done that, upgrading from one version of CodeIgniter 4 to the next
will be straightforward.</p>
<p>The “lean, mean and simple” philosophy has been retained, but the
implementation has a lot of differences, compared to CodeIgniter 3.</p>
<p>There is no 12-step checklist for upgrading. Instead, start with a copy
of CodeIgniter 4 in a new project folder,
<a class="reference internal" href="index.html"><span class="doc">however you wish to install and use it</span></a>,
and then convert and integrate your app components.
We’ll try to point out the most important considerations here.</p>
<p>Not all of the CI3 libraries have been ported or rewritten for CI4!
See the threads in the <a class="reference external" href="https://forum.codeigniter.com/forum-33.html">CodeIgniter 4 Roadmap</a>
subforum for an up-to-date list!</p>
<p><strong>Do read the user guide</strong> before embarking on a project conversion!</p>
<p><strong>Downloads</strong></p>
<ul class="simple">
<li>CI4 is still available as a ready-to-run zip or tarball, which
includes the user guide (though in the <cite>docs</cite> subfolder)</li>
<li>It can also be installed using Composer</li>
</ul>
<p><strong>Namespaces</strong></p>
<ul class="simple">
<li>CI4 is built for PHP7.3+, and everything in the framework is namespaced, except for the helpers.</li>
</ul>
<p><strong>Application Structure</strong></p>
<ul class="simple">
<li>The <code class="docutils literal notranslate"><span class="pre">application</span></code> folder is renamed as <code class="docutils literal notranslate"><span class="pre">app</span></code> and
the framework still has <code class="docutils literal notranslate"><span class="pre">system</span></code> folders, with the same
interpretation as before</li>
<li>The framework now provides for a <code class="docutils literal notranslate"><span class="pre">public</span></code> folder, intended as the document
root for your app</li>
<li>There is also a <code class="docutils literal notranslate"><span class="pre">writable</span></code> folder, to hold cache data, logs, and session data</li>
<li>The <code class="docutils literal notranslate"><span class="pre">app</span></code> folder looks very similar to <code class="docutils literal notranslate"><span class="pre">application</span></code> for CI3, with some
name changes, and some subfolders
moved to the <code class="docutils literal notranslate"><span class="pre">writable</span></code> folder</li>
<li>There is no longer a nested <code class="docutils literal notranslate"><span class="pre">application/core</span></code> folder, as we have
a different mechanism for extending framework components (see below)</li>
</ul>
<p><strong>Class loading</strong></p>
<ul class="simple">
<li>There is no longer a CodeIgniter “superobject”, with framework component
references magically injected as properties of your controller</li>
<li>Classes are instantiated where needed, and components are managed
by <code class="docutils literal notranslate"><span class="pre">Services</span></code></li>
<li>The class loader automatically handles PSR4 style class locating,
within the <code class="docutils literal notranslate"><span class="pre">App</span></code> (application) and <code class="docutils literal notranslate"><span class="pre">CodeIgniter</span></code> (i.e., system) top level
namespaces; with composer autoloading support, and even using educated
guessing to find your models and libraries if they are in the right
folder even though not namespaced</li>
<li>You can configure the class loading to support whatever application structure
you are most comfortable with, including the “HMVC” style</li>
</ul>
<p><strong>Controllers</strong></p>
<ul class="simple">
<li>Controllers extend \CodeIgniter\Controller instead of CI_Controller</li>
<li>They don’t use a constructor any more (to invoke CI “magic”) unless
that is part of a base controller you make</li>
<li>CI provides <code class="docutils literal notranslate"><span class="pre">Request</span></code> and <code class="docutils literal notranslate"><span class="pre">Response</span></code> objects for you to work with -
more powerful than the CI3-way</li>
<li>If you want a base controller (MY_Controller in CI3), make it
where you like, e.g., BaseController extends Controller, and then
have your controllers extend it</li>
</ul>
<p><strong>Models</strong></p>
<ul class="simple">
<li>Models extend \CodeIgniter\Model instead of CI_Model</li>
<li>The CI4 model has much more functionality, including automatic
database connection, basic CRUD, in-model validation, and
automatic pagination</li>
<li>CI4 also has the <code class="docutils literal notranslate"><span class="pre">Entity</span></code> class you can build on, for
richer data mapping to your database tables</li>
<li>Instead of CI3’s <code class="docutils literal notranslate"><span class="pre">$this-&gt;load-&gt;model(x);</span></code>, you would now use
<code class="docutils literal notranslate"><span class="pre">$this-&gt;x</span> <span class="pre">=</span> <span class="pre">new</span> <span class="pre">X();</span></code>, following namespaced conventions for your component</li>
</ul>
<p><strong>Views</strong></p>
<ul class="simple">
<li>Your views look much like before, but they are invoked differently …
instead of CI3’s <code class="docutils literal notranslate"><span class="pre">$this-&gt;load-&gt;view(x);</span></code> you can use <code class="docutils literal notranslate"><span class="pre">echo</span> <span class="pre">view(x);</span></code></li>
<li>CI4 supports view “cells”, to build your response in pieces</li>
<li>The template parser is still there, but substantially
enhanced</li>
</ul>
<p><strong>Libraries</strong></p>
<ul class="simple">
<li>Your app classes can still go inside <code class="docutils literal notranslate"><span class="pre">app/Libraries</span></code>, but they
don’t have to</li>
<li>Instead of CI3’s <code class="docutils literal notranslate"><span class="pre">$this-&gt;load-&gt;library(x);</span></code> you can now use
<code class="docutils literal notranslate"><span class="pre">$this-&gt;x</span> <span class="pre">=</span> <span class="pre">new</span> <span class="pre">X();</span></code>, following namespaced conventions for your
component</li>
</ul>
<p><strong>Helpers</strong></p>
<ul class="simple">
<li>Helpers are pretty much the same as before, though some have been simplified</li>
</ul>
<p><strong>Events</strong></p>
<ul class="simple">
<li>Hooks have been replaced by Events</li>
<li>Instead of CI3’s <code class="docutils literal notranslate"><span class="pre">$hook['post_controller_constructor']</span></code> you now use <code class="docutils literal notranslate"><span class="pre">Events::on('post_controller_constructor',</span> <span class="pre">['MyClass',</span> <span class="pre">'MyFunction']);</span></code>, with the namespace <code class="docutils literal notranslate"><span class="pre">CodeIgniter\Events\Events;</span></code></li>
<li>Events are always enabled, and are available globally</li>
</ul>
<p><strong>Extending the framework</strong></p>
<ul class="simple">
<li>You don’t need a <code class="docutils literal notranslate"><span class="pre">core</span></code> folder to hold <code class="docutils literal notranslate"><span class="pre">MY_...</span></code> framework
component extensions or replacements</li>
<li>You don’t need <code class="docutils literal notranslate"><span class="pre">MY_x</span></code> classes inside your libraries folder
to extend or replace CI4 pieces</li>
<li>Make any such classes where you like, and add appropriate
service methods in <code class="docutils literal notranslate"><span class="pre">app/Config/Services.php</span></code> to load
your components instead of the default ones</li>
</ul>
</div>


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