diff --git a/psalm.xml b/psalm.xml
index 7d325c7f..4096cab8 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -30,6 +30,14 @@
         <RedundantPropertyInitializationCheck errorLevel="suppress"/>
         <MissingConstructor errorLevel="info"/>
 
+        <!-- There are some include statements in tests that psalm cannot resolve,
+        but they are correct, part of the mocked Moodle environment. -->
+        <UnresolvableInclude>
+            <errorLevel type="suppress">
+                <directory name="tests"/>
+            </errorLevel>
+        </UnresolvableInclude>
+
         <!-- Test classes are, apparently, not used but, for sure, they are used by PHPUnit. -->
         <UnusedClass>
             <errorLevel type="suppress">
diff --git a/tests/Bridge/MoodleTest.php b/tests/Bridge/MoodleTest.php
index cb0a5e57..c1eed1d7 100644
--- a/tests/Bridge/MoodleTest.php
+++ b/tests/Bridge/MoodleTest.php
@@ -13,12 +13,116 @@
 namespace MoodlePluginCI\Tests\Bridge;
 
 use MoodlePluginCI\Bridge\Moodle;
+use MoodlePluginCI\Tests\FilesystemTestCase;
 
-class MoodleTest extends \PHPUnit\Framework\TestCase
+/**
+ * Test the \MoodlePluginCI\Bridge\Moodle class.
+ *
+ * Note that various unit tests are run in a separate process to avoid
+ * conflicts with global state when including Moodle's config.php or
+ * core_component (mocked) class.
+ *
+ * @covers \MoodlePluginCI\Bridge\Moodle
+ */
+class MoodleTest extends FilesystemTestCase
 {
-    public function testGetBranch(): void
+    /**
+     * @runInSeparateProcess
+     */
+    public function testGetConfigExists(): void
+    {
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+        $this->fs->copy(__DIR__ . '/../Fixture/tiny-config.php', $this->tempDir . '/config.php');
+        $moodle = new Moodle($this->tempDir);
+
+        $this->assertSame('exists', $moodle->getConfig('exists'));
+    }
+
+    /**
+     * @runInSeparateProcess
+     */
+    public function testGetConfigNotExists(): void
+    {
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+        $this->fs->copy(__DIR__ . '/../Fixture/tiny-config.php', $this->tempDir . '/config.php');
+        $moodle = new Moodle($this->tempDir);
+
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('Failed to find $CFG->notexists in Moodle config file');
+        $moodle->getConfig('notexists');
+    }
+
+    /**
+     * @runInSeparateProcess
+     */
+    public function testGetConfigNoConfigFile(): void
+    {
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+        $moodle = new Moodle($this->tempDir);
+
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('Failed to find Moodle config file');
+        $moodle->getConfig('wwwroot');
+    }
+
+    /**
+     * @runInSeparateProcess
+     */
+    public function testComponentInstallDirectoryExists(): void
     {
-        $moodle = new Moodle(__DIR__ . '/../Fixture/moodle');
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+        $this->fs->copy(__DIR__ . '/../Fixture/tiny-config.php', $this->tempDir . '/config.php');
+        $moodle = new Moodle($this->tempDir);
+
+        // Initially we tried to mock the core_component class with Mockery, and everything
+        // worked fine, but the use of Reflection that we have in the code to be tested.
+        // Hence, we have changed to load a minimal core_component (Fixture/moodle/lib/classes/component.php)
+        // class that will be used instead of the (core) original.
+        require_once $this->tempDir . '/lib/classes/component.php';
+
+        $this->assertSame('/path/to/mod/test', $moodle->getComponentInstallDirectory('mod_test'));
+    }
+
+    /**
+     * @runInSeparateProcess
+     */
+    public function testComponentInstallDirectoryNotExists(): void
+    {
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+        $this->fs->copy(__DIR__ . '/../Fixture/tiny-config.php', $this->tempDir . '/config.php');
+        $moodle = new Moodle($this->tempDir);
+
+        // Initially we tried to mock the core_component class with Mockery, and everything
+        // worked fine, but the use of Reflection that we have in the code to be tested.
+        // Hence, we have changed to load a minimal core_component (Fixture/moodle/lib/classes/component.php)
+        // class that will be used instead of the (core) original.
+        require_once $this->tempDir . '/lib/classes/component.php';
+
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('The component unknown_test has an unknown plugin type of unknown');
+        $moodle->getComponentInstallDirectory('unknown_test');
+    }
+
+    public function testGetBranchCorrect(): void
+    {
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+        $moodle = new Moodle($this->tempDir);
+
         $this->assertSame(39, $moodle->getBranch());
     }
+
+    public function testGetBranchIncorrect(): void
+    {
+        $this->fs->mirror(__DIR__ . '/../Fixture/moodle', $this->tempDir);
+
+        // Let's edit the version.php file to convert the correct (string) branch into incorrect (integer) branch.
+        $contents = file_get_contents($this->tempDir . '/version.php');
+        $contents = preg_replace("/'39'/", '39', $contents);
+        file_put_contents($this->tempDir . '/version.php', $contents);
+
+        $moodle = new Moodle($this->tempDir);
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('Failed to find Moodle branch version');
+        $moodle->getBranch();
+    }
 }
diff --git a/tests/Command/MessDetectorCommandTest.php b/tests/Command/MessDetectorCommandTest.php
index 60d02dbb..d116f52c 100644
--- a/tests/Command/MessDetectorCommandTest.php
+++ b/tests/Command/MessDetectorCommandTest.php
@@ -52,6 +52,16 @@ public function testExecute(): void
         $this->assertSame(0, $commandTester->getStatusCode());
     }
 
+    public function testExecuteWithProblems(): void
+    {
+        $commandTester = $this->executeCommand(__DIR__ . '/../Fixture/phpmd');
+
+        $this->assertSame(0, $commandTester->getStatusCode()); // PHPMD always return 0, no matter the violations/errors.
+        $this->assertStringContainsString('Constant testConst should be defined in uppercase', $commandTester->getDisplay());
+        $this->assertStringContainsString('long variable names like $reallyVeryLongVariableName', $commandTester->getDisplay());
+        $this->assertStringContainsString('Unexpected end of token stream in file', $commandTester->getDisplay());
+    }
+
     public function testExecuteNoFiles(): void
     {
         // Just random directory with no PHP files.
diff --git a/tests/Fixture/moodle/lib/classes/component.php b/tests/Fixture/moodle/lib/classes/component.php
new file mode 100644
index 00000000..ccd5279a
--- /dev/null
+++ b/tests/Fixture/moodle/lib/classes/component.php
@@ -0,0 +1,41 @@
+<?php
+
+/*
+ * This file is part of the Moodle Plugin CI package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ *
+ * @copyright 2024 onwards Eloy Lafuente (stronk7) {@link https://stronk7.com}
+ * @license https://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+/**
+ * Minimal core_component core class.
+ *
+ * This is a minimal fixture of the Moodle core_component class, just to be able
+ * to use it in the tests, only including the methods that we are going to need to
+ * invoke.
+ *
+ * Note that we haven't been able to use Mockery for this, because the code to be
+ * tested is using Reflection on this class, so we cannot aliases/overload or
+ * use similar strategies to mock it.
+ */
+class core_component {
+
+    public static function get_component_directory($component) {
+        return '/path/to/' . str_replace('_', '/', $component);
+    }
+
+    public static function normalize_component($component) {
+        return explode('_', $component, 2);
+    }
+
+    protected static function fetch_plugintypes() {
+        return [
+            ['mod' => '/path/to/mod', 'local' =>  '/path/to/local'],
+            [], // We don't need this.
+            [], // We don't need this.
+        ];
+    }
+}
diff --git a/tests/Fixture/phpmd/Problems.php b/tests/Fixture/phpmd/Problems.php
new file mode 100644
index 00000000..a396113e
--- /dev/null
+++ b/tests/Fixture/phpmd/Problems.php
@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * This file is part of the Moodle Plugin CI package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ *
+ * @copyright 2024 onwards Eloy Lafuente (stronk7) {@link https://stronk7.com}
+ * @license https://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+/**
+ * This fixture class is used to test the PHPMD command.
+ *
+ * It contains 2 PHPMD naming violations and 1 parsing error.
+ */
+class Problems {
+    const testConst = 1;
+    protected $reallyVeryLongVariableName = true;
+}
+
+class unfinishedFixtureClass {
diff --git a/tests/Fixture/tiny-config.php b/tests/Fixture/tiny-config.php
new file mode 100644
index 00000000..17176c01
--- /dev/null
+++ b/tests/Fixture/tiny-config.php
@@ -0,0 +1,33 @@
+<?php // Moodle configuration file
+
+unset($CFG);
+global $CFG;
+$CFG = new stdClass();
+
+$CFG->dbtype    = 'mysqli';
+$CFG->dblibrary = 'native';
+$CFG->dbhost    = 'localhost';
+$CFG->dbname    = 'moodle';
+$CFG->dbuser    = 'root';
+$CFG->dbpass    = '';
+$CFG->prefix    = 'mdl_';
+$CFG->dboptions = [
+    'dbport' => '',
+];
+
+$CFG->wwwroot  = 'http://localhost/moodle';
+$CFG->dataroot = '/path/to/moodledata';
+$CFG->admin    = 'admin';
+
+$CFG->directorypermissions = 02777;
+
+// Show debugging messages.
+$CFG->debug        = (E_ALL | E_STRICT);
+$CFG->debugdisplay = 1;
+
+// Extra config.
+$CFG->exists = 'exists';
+
+// require_once(__DIR__.'/lib/setup.php');
+// There is no php closing tag in this file,
+// it is intentional because it prevents trailing whitespace problems!