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CodeIgniter Overview
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Application vs System folders

Last modified on: September 7, 2025

<h1 data-start="508" data-end="556">📝 CodeIgniter Application vs System Folders</h1>
<h2 data-start="558" data-end="578">🔎 Introduction</h2>
<p data-start="579" data-end="647">CodeIgniter organizes your project into two main types of folders:</p>
<ol data-start="649" data-end="798">
<li data-start="649" data-end="720">
<p data-start="652" data-end="720"><strong data-start="652" data-end="683">Application Folder (<code data-start="674" data-end="680">app/</code>)</strong> &ndash; Where your project code resides.</p>
</li>
<li data-start="721" data-end="798">
<p data-start="724" data-end="798"><strong data-start="724" data-end="753">System Folder (<code data-start="741" data-end="750">system/</code>)</strong> &ndash; Where the framework&rsquo;s core files reside.</p>
</li>
</ol>
<p data-start="800" data-end="878">This separation makes CodeIgniter <strong data-start="834" data-end="875">lightweight, secure, and maintainable</strong>.</p>
<hr data-start="880" data-end="883" />
<h2 data-start="885" data-end="923">📂 1. Application Folder (<code data-start="914" data-end="920">app/</code>)</h2>
<h3 data-start="925" data-end="939">Purpose:</h3>
<ul data-start="940" data-end="1040">
<li data-start="940" data-end="987">
<p data-start="942" data-end="987">Holds <strong data-start="948" data-end="984">all your custom application code</strong>.</p>
</li>
<li data-start="988" data-end="1040">
<p data-start="990" data-end="1040">Developers <strong data-start="1001" data-end="1018">actively work</strong> inside this folder.</p>
</li>
</ul>
<h3 data-start="1042" data-end="1057">Contents:</h3>
<div class="_tableContainer_1rjym_1">
<div class="_tableWrapper_1rjym_13 group flex w-fit flex-col-reverse" tabindex="-1">
<table class="w-fit min-w-(--thread-content-width)" data-start="1059" data-end="1670">
<thead data-start="1059" data-end="1092">
<tr data-start="1059" data-end="1092">
<th data-start="1059" data-end="1081" data-col-size="sm">Folder/File</th>
<th data-start="1081" data-end="1092" data-col-size="md">Purpose</th>
</tr>
</thead>
<tbody data-start="1126" data-end="1670">
<tr data-start="1126" data-end="1201">
<td data-start="1126" data-end="1147" data-col-size="sm"><code data-start="1128" data-end="1142">Controllers/</code></td>
<td data-start="1147" data-end="1201" data-col-size="md">Handles user requests and connects Models &amp; Views.</td>
</tr>
<tr data-start="1202" data-end="1274">
<td data-start="1202" data-end="1223" data-col-size="sm"><code data-start="1204" data-end="1213">Models/</code></td>
<td data-start="1223" data-end="1274" data-col-size="md">Interacts with the database and business logic.</td>
</tr>
<tr data-start="1275" data-end="1338">
<td data-start="1275" data-end="1296" data-col-size="sm"><code data-start="1277" data-end="1285">Views/</code></td>
<td data-start="1296" data-end="1338" data-col-size="md">HTML/PHP templates displayed to users.</td>
</tr>
<tr data-start="1339" data-end="1418">
<td data-start="1339" data-end="1360" data-col-size="sm"><code data-start="1341" data-end="1350">Config/</code></td>
<td data-start="1360" data-end="1418" data-col-size="md">Configuration files like database, routes, email, etc.</td>
</tr>
<tr data-start="1419" data-end="1470">
<td data-start="1419" data-end="1440" data-col-size="sm"><code data-start="1421" data-end="1431">Helpers/</code></td>
<td data-start="1440" data-end="1470" data-col-size="md">Custom reusable functions.</td>
</tr>
<tr data-start="1471" data-end="1535">
<td data-start="1471" data-end="1492" data-col-size="sm"><code data-start="1473" data-end="1483">Filters/</code></td>
<td data-start="1492" data-end="1535" data-col-size="md">Middleware-like processing of requests.</td>
</tr>
<tr data-start="1536" data-end="1605">
<td data-start="1536" data-end="1557" data-col-size="sm"><code data-start="1538" data-end="1549">Language/</code></td>
<td data-start="1557" data-end="1605" data-col-size="md">For localization and multi-language support.</td>
</tr>
<tr data-start="1606" data-end="1670">
<td data-start="1606" data-end="1627" data-col-size="sm"><code data-start="1608" data-end="1620">Libraries/</code></td>
<td data-start="1627" data-end="1670" data-col-size="md">Custom libraries created by developers.</td>
</tr>
</tbody>
</table>
</div>
</div>
<h3 data-start="1672" data-end="1689">Key Points:</h3>
<ul data-start="1690" data-end="1843">
<li data-start="1690" data-end="1721">
<p data-start="1692" data-end="1721"><strong data-start="1692" data-end="1719">Editable by developers.</strong></p>
</li>
<li data-start="1722" data-end="1786">
<p data-start="1724" data-end="1786">Represents the <strong data-start="1739" data-end="1763">business logic layer</strong> of your application.</p>
</li>
<li data-start="1787" data-end="1843">
<p data-start="1789" data-end="1843">Safe to modify without affecting the framework core.</p>
</li>
</ul>
<p data-start="1845" data-end="1859"><strong data-start="1845" data-end="1857">Example:</strong></p>
<div class="contain-inline-size rounded-2xl relative bg-token-sidebar-surface-primary">
<div class="sticky top-9">
<div class="absolute end-0 bottom-0 flex h-9 items-center pe-2">
<div class="bg-token-bg-elevated-secondary text-token-text-secondary flex items-center gap-4 rounded-sm px-2 font-sans text-xs">
<pre class="language-php"><code>app/Controllers/Home.php
app/Models/UserModel.php
app/Views/home_view.php
</code></pre>
</div>
</div>
</div>
</div>
<hr data-start="1946" data-end="1949" />
<h2 data-start="1951" data-end="1987">📂 2. System Folder (<code data-start="1975" data-end="1984">system/</code>)</h2>
<h3 data-start="1989" data-end="2003">Purpose:</h3>
<ul data-start="2004" data-end="2142">
<li data-start="2004" data-end="2054">
<p data-start="2006" data-end="2054">Contains <strong data-start="2015" data-end="2051">CodeIgniter framework core files</strong>.</p>
</li>
<li data-start="2055" data-end="2142">
<p data-start="2057" data-end="2142">Responsible for <strong data-start="2073" data-end="2139">routing, request handling, database connection, and MVC engine</strong>.</p>
</li>
</ul>
<h3 data-start="2144" data-end="2159">Contents:</h3>
<div class="_tableContainer_1rjym_1">
<div class="_tableWrapper_1rjym_13 group flex w-fit flex-col-reverse" tabindex="-1">
<table class="w-fit min-w-(--thread-content-width)" data-start="2161" data-end="2518">
<thead data-start="2161" data-end="2193">
<tr data-start="2161" data-end="2193">
<th data-start="2161" data-end="2182" data-col-size="sm">Folder/File</th>
<th data-start="2182" data-end="2193" data-col-size="sm">Purpose</th>
</tr>
</thead>
<tbody data-start="2226" data-end="2518">
<tr data-start="2226" data-end="2282">
<td data-start="2226" data-end="2246" data-col-size="sm"><code data-start="2228" data-end="2235">Core/</code></td>
<td data-start="2246" data-end="2282" data-col-size="sm">Base classes and core functions.</td>
</tr>
<tr data-start="2283" data-end="2342">
<td data-start="2283" data-end="2303" data-col-size="sm"><code data-start="2285" data-end="2296">Database/</code></td>
<td data-start="2303" data-end="2342" data-col-size="sm">Database drivers and query builder.</td>
</tr>
<tr data-start="2343" data-end="2398">
<td data-start="2343" data-end="2363" data-col-size="sm"><code data-start="2345" data-end="2352">View/</code></td>
<td data-start="2363" data-end="2398" data-col-size="sm">Internal view rendering engine.</td>
</tr>
<tr data-start="2399" data-end="2456">
<td data-start="2399" data-end="2419" data-col-size="sm"><code data-start="2401" data-end="2413">Router.php</code></td>
<td data-start="2419" data-end="2456" data-col-size="sm">Handles routing of HTTP requests.</td>
</tr>
<tr data-start="2457" data-end="2518">
<td data-start="2457" data-end="2477" data-col-size="sm"><code data-start="2459" data-end="2476">CodeIgniter.php</code></td>
<td data-start="2477" data-end="2518" data-col-size="sm">Core front controller bootstrap file.</td>
</tr>
</tbody>
</table>
</div>
</div>
<h3 data-start="2520" data-end="2537">Key Points:</h3>
<ul data-start="2538" data-end="2720">
<li data-start="2538" data-end="2598">
<p data-start="2540" data-end="2598"><strong data-start="2540" data-end="2557">Do not modify</strong> &ndash; changes may break framework updates.</p>
</li>
<li data-start="2599" data-end="2655">
<p data-start="2601" data-end="2655">Acts as the <strong data-start="2613" data-end="2623">engine</strong> that powers your application.</p>
</li>
<li data-start="2656" data-end="2720">
<p data-start="2658" data-end="2720">Keeps <strong data-start="2664" data-end="2704">system files separate from app files</strong> for security.</p>
</li>
</ul>
<hr data-start="2722" data-end="2725" />
<h2 data-start="2727" data-end="2753">🔄 3. Key Differences</h2>
<div class="_tableContainer_1rjym_1">
<div class="_tableWrapper_1rjym_13 group flex w-fit flex-col-reverse" tabindex="-1">
<table class="w-fit min-w-(--thread-content-width)" data-start="2755" data-end="3449">
<thead data-start="2755" data-end="2856">
<tr data-start="2755" data-end="2856">
<th data-start="2755" data-end="2782" data-col-size="sm">Feature</th>
<th data-start="2782" data-end="2819" data-col-size="sm">Application Folder (<code data-start="2804" data-end="2810">app/</code>)</th>
<th data-start="2819" data-end="2856" data-col-size="sm">System Folder (<code data-start="2836" data-end="2845">system/</code>)</th>
</tr>
</thead>
<tbody data-start="2957" data-end="3449">
<tr data-start="2957" data-end="3055">
<td data-start="2957" data-end="2984" data-col-size="sm">Purpose</td>
<td data-start="2984" data-end="3019" data-col-size="sm">Holds custom app code</td>
<td data-start="3019" data-end="3055" data-col-size="sm">Holds framework core files</td>
</tr>
<tr data-start="3056" data-end="3152">
<td data-start="3056" data-end="3083" data-col-size="sm">Editable by Developer</td>
<td data-start="3083" data-end="3117" data-col-size="sm">✅ Yes</td>
<td data-start="3117" data-end="3152" data-col-size="sm">❌ No (should not be modified)</td>
</tr>
<tr data-start="3153" data-end="3254">
<td data-start="3153" data-end="3189" data-col-size="sm">Contains Controllers/Models/Views</td>
<td data-start="3189" data-end="3219" data-col-size="sm">✅ Yes</td>
<td data-start="3219" data-end="3254" data-col-size="sm">❌ No</td>
</tr>
<tr data-start="3255" data-end="3351">
<td data-start="3255" data-end="3282" data-col-size="sm">Contains Framework Core</td>
<td data-start="3282" data-end="3316" data-col-size="sm">❌ No</td>
<td data-start="3316" data-end="3351" data-col-size="sm">✅ Yes</td>
</tr>
<tr data-start="3352" data-end="3449">
<td data-start="3352" data-end="3379" data-col-size="sm">Security Risk if exposed</td>
<td data-start="3379" data-end="3414" data-col-size="sm">Moderate (depends on file access)</td>
<td data-start="3414" data-end="3449" data-col-size="sm">High (core framework files)</td>
</tr>
</tbody>
</table>
</div>
</div>
<hr data-start="3451" data-end="3454" />
<h2 data-start="3456" data-end="3490">🎯 4. Why Keep Them Separate?</h2>
<ol data-start="3492" data-end="3837">
<li data-start="3492" data-end="3555">
<p data-start="3495" data-end="3555"><strong data-start="3495" data-end="3508">Security:</strong> Users cannot access core files from the web.</p>
</li>
<li data-start="3556" data-end="3645">
<p data-start="3559" data-end="3645"><strong data-start="3559" data-end="3579">Maintainability:</strong> Upgrading CodeIgniter is easier if core files remain untouched.</p>
</li>
<li data-start="3646" data-end="3751">
<p data-start="3649" data-end="3751"><strong data-start="3649" data-end="3666">Organization:</strong> Developers only focus on <code data-start="3692" data-end="3698">app/</code>, while <code data-start="3706" data-end="3715">system/</code> handles the framework operations.</p>
</li>
<li data-start="3752" data-end="3837">
<p data-start="3755" data-end="3837"><strong data-start="3755" data-end="3771">Scalability:</strong> Multiple applications can share one <code data-start="3808" data-end="3817">system/</code> folder if needed.</p>
</li>
</ol>
<hr data-start="3839" data-end="3842" />
<h2 data-start="3844" data-end="3877">🖼️ 5. Visual Representation</h2>
<div class="contain-inline-size rounded-2xl relative bg-token-sidebar-surface-primary">
<div class="sticky top-9">
<div class="absolute end-0 bottom-0 flex h-9 items-center pe-2">
<div class="bg-token-bg-elevated-secondary text-token-text-secondary flex items-center gap-4 rounded-sm px-2 font-sans text-xs">
<pre class="language-php"><code>CodeIgniter Project
├── app/        &rarr; Your application logic (Controllers, Models, Views)
└── system/     &rarr; Framework engine (Core, Database, Router, View)
</code></pre>
</div>
</div>
</div>
</div>
<p data-start="4044" data-end="4063"><strong data-start="4044" data-end="4061">Flow Example:</strong></p>
<ul data-start="4064" data-end="4182">
<li data-start="4064" data-end="4182">
<p data-start="4066" data-end="4182">User request &rarr; <code data-start="4081" data-end="4092">index.php</code> &rarr; <code data-start="4095" data-end="4104">system/</code> (framework engine) &rarr; <code data-start="4126" data-end="4142">app/Controller</code> &rarr; <code data-start="4145" data-end="4156">app/Model</code> &rarr; <code data-start="4159" data-end="4169">app/View</code> &rarr; Response</p>
</li>
</ul>
<hr data-start="4184" data-end="4187" />
<h2 data-start="4189" data-end="4206">✅ Conclusion</h2>
<ul data-start="4208" data-end="4531">
<li data-start="4208" data-end="4308">
<p data-start="4210" data-end="4308"><strong data-start="4210" data-end="4241">Application Folder (<code data-start="4232" data-end="4238">app/</code>)</strong>: Editable by developers, contains your project&rsquo;s business logic.</p>
</li>
<li data-start="4309" data-end="4408">
<p data-start="4311" data-end="4408"><strong data-start="4311" data-end="4340">System Folder (<code data-start="4328" data-end="4337">system/</code>)</strong>: Core framework, should never be modified, powers the MVC engine.</p>
</li>
<li data-start="4409" data-end="4531">
<p data-start="4411" data-end="4531">Keeping these folders separate ensures <strong data-start="4450" data-end="4496">security, maintainability, and scalability</strong> of your CodeIgniter application.</p>
</li>
</ul>

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