diff --git a/src/Parser/StatementFilter.php b/src/Parser/StatementFilter.php
index 04649de6..4d5bb4e2 100644
--- a/src/Parser/StatementFilter.php
+++ b/src/Parser/StatementFilter.php
@@ -14,9 +14,11 @@
 
 use PhpParser\Node\Expr\Array_;
 use PhpParser\Node\Expr\Assign;
+use PhpParser\Node\Expr\FuncCall;
 use PhpParser\Node\Expr\PropertyFetch;
 use PhpParser\Node\Expr\Variable;
 use PhpParser\Node\Identifier;
+use PhpParser\Node\Name;
 use PhpParser\Node\Scalar\String_;
 use PhpParser\Node\Stmt;
 use PhpParser\Node\Stmt\Class_;
@@ -116,6 +118,26 @@ public function filterAssignments(array $statements): array
         return $assigns;
     }
 
+    /**
+     * Extract all the function call expressions from the statements.
+     *
+     * @param Stmt[] $statements
+     *
+     * @return FuncCall[]
+     */
+    public function filterFunctionCalls(array $statements): array
+    {
+        $calls = [];
+        foreach ($statements as $statement) {
+            // Only expressions that are function calls.
+            if ($statement instanceof Expression && $statement->expr instanceof FuncCall) {
+                $calls[] = $statement->expr;
+            }
+        }
+
+        return $calls;
+    }
+
     /**
      * Find first variable assignment with a given name.
      *
diff --git a/src/PluginValidate/Finder/FileTokens.php b/src/PluginValidate/Finder/FileTokens.php
index ca0014e3..4f5b08f4 100644
--- a/src/PluginValidate/Finder/FileTokens.php
+++ b/src/PluginValidate/Finder/FileTokens.php
@@ -29,6 +29,13 @@ class FileTokens
      */
     public string $file;
 
+    /**
+     * Not found error hint.
+     *
+     * @var string
+     */
+    public string $hint = '';
+
     /**
      * @param string $file
      */
@@ -59,6 +66,16 @@ public function hasTokens(): bool
         return !empty($this->tokens);
     }
 
+    /**
+     * Do we have any hint?
+     *
+     * @return bool
+     */
+    public function hasHint(): bool
+    {
+        return !empty($this->hint);
+    }
+
     /**
      * @param Token $token
      *
@@ -166,4 +183,18 @@ public function resetTokens(): void
             $token->reset();
         }
     }
+
+    /**
+     * Not found error additional information guiding user how to fix it (optional).
+     *
+     * @param string $hint
+     *
+     * @return self
+     */
+    public function notFoundHint(string $hint): self
+    {
+        $this->hint = $hint;
+
+        return $this;
+    }
 }
diff --git a/src/PluginValidate/Finder/FunctionCallFinder.php b/src/PluginValidate/Finder/FunctionCallFinder.php
new file mode 100644
index 00000000..609ec1e3
--- /dev/null
+++ b/src/PluginValidate/Finder/FunctionCallFinder.php
@@ -0,0 +1,37 @@
+<?php
+
+/*
+ * This file is part of the Moodle Plugin CI package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ *
+ * Copyright (c) 2024 Moodle Pty Ltd <support@moodle.com>
+ * License http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace MoodlePluginCI\PluginValidate\Finder;
+
+use PhpParser\Node\Name;
+
+/**
+ * Finds function call.
+ */
+class FunctionCallFinder extends AbstractParserFinder
+{
+    public function getType(): string
+    {
+        return 'function call';
+    }
+
+    public function findTokens($file, FileTokens $fileTokens): void
+    {
+        $statements = $this->parser->parseFile($file);
+
+        foreach ($this->filter->filterFunctionCalls($statements) as $funccall) {
+            if ($funccall->name instanceof Name) {
+                $fileTokens->compare((string) $funccall->name);
+            }
+        }
+    }
+}
diff --git a/src/PluginValidate/PluginValidate.php b/src/PluginValidate/PluginValidate.php
index 93c03b82..be9c5ee9 100644
--- a/src/PluginValidate/PluginValidate.php
+++ b/src/PluginValidate/PluginValidate.php
@@ -17,6 +17,7 @@
 use MoodlePluginCI\PluginValidate\Finder\ClassFinder;
 use MoodlePluginCI\PluginValidate\Finder\FileTokens;
 use MoodlePluginCI\PluginValidate\Finder\FinderInterface;
+use MoodlePluginCI\PluginValidate\Finder\FunctionCallFinder;
 use MoodlePluginCI\PluginValidate\Finder\FunctionFinder;
 use MoodlePluginCI\PluginValidate\Finder\LangFinder;
 use MoodlePluginCI\PluginValidate\Finder\TableFinder;
@@ -98,6 +99,9 @@ public function addMessagesFromTokens(string $type, FileTokens $fileTokens): voi
                 $this->addSuccess(sprintf('In %s, found %s %s', $fileTokens->file, $type, implode(' OR ', $token->tokens)));
             } else {
                 $this->addError(sprintf('In %s, failed to find %s %s', $fileTokens->file, $type, implode(' OR ', $token->tokens)));
+                if ($fileTokens->hasHint()) {
+                    $this->addError(sprintf('Hint: %s', $fileTokens->hint));
+                }
             }
         }
     }
@@ -115,6 +119,7 @@ public function verifyRequirements(): void
         $this->findRequiredTokens(new TableFinder(), [$this->requirements->getRequiredTables()]);
         $this->findRequiredTokens(new TablePrefixFinder(), [$this->requirements->getRequiredTablePrefix()]);
         $this->findRequiredTokens(new BehatTagFinder(), $this->requirements->getRequiredBehatTags());
+        $this->findRequiredTokens(new FunctionCallFinder(), $this->requirements->getRequiredFunctionCalls());
     }
 
     /**
diff --git a/src/PluginValidate/Requirements/AbstractRequirements.php b/src/PluginValidate/Requirements/AbstractRequirements.php
index 9978dd2f..1f5670b8 100644
--- a/src/PluginValidate/Requirements/AbstractRequirements.php
+++ b/src/PluginValidate/Requirements/AbstractRequirements.php
@@ -64,6 +64,13 @@ protected function behatTagsFactory(array $tags): array
         return $fileTokens;
     }
 
+    /**
+     * Required function calls.
+     *
+     * @return FileTokens[]
+     */
+    abstract public function getRequiredFunctionCalls(): array;
+
     /**
      * An array of required files, paths are relative to the plugin directory.
      *
diff --git a/src/PluginValidate/Requirements/FilterRequirements.php b/src/PluginValidate/Requirements/FilterRequirements.php
index 47738373..78daf9d5 100644
--- a/src/PluginValidate/Requirements/FilterRequirements.php
+++ b/src/PluginValidate/Requirements/FilterRequirements.php
@@ -48,4 +48,15 @@ public function getRequiredStrings(): FileTokens
     {
         return FileTokens::create($this->getLangFile())->mustHave('filtername');
     }
+
+    public function getRequiredFunctionCalls(): array
+    {
+        if ($this->moodleVersion <= 404) {
+            return [];
+        }
+
+        return [
+            FileTokens::create('filter.php')->mustHave('class_alias')->notFoundHint('https://moodledev.io/docs/4.5/devupdate#filter-plugins'),
+        ];
+    }
 }
diff --git a/src/PluginValidate/Requirements/GenericRequirements.php b/src/PluginValidate/Requirements/GenericRequirements.php
index d1dccf4f..d6cd412b 100644
--- a/src/PluginValidate/Requirements/GenericRequirements.php
+++ b/src/PluginValidate/Requirements/GenericRequirements.php
@@ -46,6 +46,11 @@ public function getRequiredClasses(): array
         return [];
     }
 
+    public function getRequiredFunctionCalls(): array
+    {
+        return [];
+    }
+
     public function getRequiredStrings(): FileTokens
     {
         return FileTokens::create($this->getLangFile())->mustHave('pluginname');
diff --git a/tests/Fixture/moodle-local_ci/lib.php b/tests/Fixture/moodle-local_ci/lib.php
index 75451d04..2a000d7d 100644
--- a/tests/Fixture/moodle-local_ci/lib.php
+++ b/tests/Fixture/moodle-local_ci/lib.php
@@ -28,6 +28,8 @@
 // set then can check for anything, like CUSTOM-123 or https://github.com
 // or whatever.
 
+defined('MOODLE_INTERNAL') || die();
+
 /**
  * Add
  *
@@ -72,3 +74,6 @@ public function add($a, $b) {
         return $a + $b;
     }
 }
+
+// Call function.
+local_ci_subtract(1, 2);
diff --git a/tests/PluginValidate/Finder/FunctionCallFinderTest.php b/tests/PluginValidate/Finder/FunctionCallFinderTest.php
new file mode 100644
index 00000000..69f44e2b
--- /dev/null
+++ b/tests/PluginValidate/Finder/FunctionCallFinderTest.php
@@ -0,0 +1,44 @@
+<?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 (c) 2024 Moodle Pty Ltd <support@moodle.com>
+ * License http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace MoodlePluginCI\Tests\PluginValidate\Finder;
+
+use MoodlePluginCI\PluginValidate\Finder\FileTokens;
+use MoodlePluginCI\PluginValidate\Finder\FunctionCallFinder;
+
+class FunctionCallFinderTest extends \PHPUnit\Framework\TestCase
+{
+    public function testFindTokens()
+    {
+        $file       = __DIR__ . '/../../Fixture/moodle-local_ci/lib.php';
+        $fileTokens = FileTokens::create('lib.php')->mustHave('local_ci_subtract');
+
+        $finder = new FunctionCallFinder();
+        $finder->findTokens($file, $fileTokens);
+
+        $this->assertTrue($fileTokens->hasFoundAllTokens());
+        $this->assertFalse($fileTokens->hasHint());
+    }
+
+    public function testFindTokensNotFound()
+    {
+        $file       = __DIR__ . '/../../Fixture/moodle-local_ci/lib.php';
+        $fileTokens = FileTokens::create('lib.php')->mustHave('exit')->notFoundHint('Exit not found');
+
+        $finder = new FunctionCallFinder();
+        $finder->findTokens($file, $fileTokens);
+
+        $this->assertFalse($fileTokens->hasFoundAllTokens());
+        $this->assertTrue($fileTokens->hasHint());
+        $this->assertSame('Exit not found', $fileTokens->hint);
+    }
+}