diff --git a/README.md b/README.md
index fe5ab82..edc0ed1 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,8 @@ Basic usage
 -----------
 
 If all you want is to scan a directory and extract a classmap with all
-classes/interfaces/traits/enums mapped to their paths, you can simply use:
+classes/interfaces/traits/enums (and named `extension` declarations from the
+proposed Extension Methods RFC) mapped to their paths, you can simply use:
 
 
 ```php
diff --git a/src/PhpFileCleaner.php b/src/PhpFileCleaner.php
index 731f698..fd11d80 100644
--- a/src/PhpFileCleaner.php
+++ b/src/PhpFileCleaner.php
@@ -20,7 +20,7 @@
  */
 class PhpFileCleaner
 {
-    /** @var array<array{name: string, length: int, pattern: non-empty-string}> */
+    /** @var array<string, list<array{name: string, length: int, pattern: non-empty-string}>> */
     private static $typeConfig;
 
     /** @var non-empty-string */
@@ -52,11 +52,22 @@ class PhpFileCleaner
      */
     public static function setTypeConfig(array $types): void
     {
+        self::$typeConfig = [];
+
         foreach ($types as $type) {
-            self::$typeConfig[$type[0]] = [
+            if ('extension' === $type) {
+                // extension declarations are only recognized with the full "Name on Target $var" tail, both to
+                // rule out false positives ("extension" is a valid symbol name) and because the early-return
+                // in clean() must keep everything the parser regex needs to see
+                $pattern = '{.\b(?<![\$:>])extension\s++[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+\s++on\s++\\\\?[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\s*+\\\\\s*+[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)*+\s*+\$[a-zA-Z_\x7f-\xff]}Ais';
+            } else {
+                $pattern = '{.\b(?<![\$:>])'.$type.'\s++[a-zA-Z_\x7f-\xff:][a-zA-Z0-9_\x7f-\xff:\-]*+}Ais';
+            }
+
+            self::$typeConfig[$type[0]][] = [
                 'name' => $type,
                 'length' => \strlen($type),
-                'pattern' => '{.\b(?<![\$:>])'.$type.'\s++[a-zA-Z_\x7f-\xff:][a-zA-Z0-9_\x7f-\xff:\-]*+}Ais',
+                'pattern' => $pattern,
             ];
         }
 
@@ -118,12 +129,13 @@ public function clean(): string
                 }
 
                 if ($this->maxMatches === 1 && isset(self::$typeConfig[$char])) {
-                    $type = self::$typeConfig[$char];
-                    if (
-                        substr($this->contents, $this->index, $type['length']) === $type['name']
-                        && Preg::isMatch($type['pattern'], $this->contents, $match, 0, $this->index - 1)
-                    ) {
-                        return $clean . $match[0];
+                    foreach (self::$typeConfig[$char] as $type) {
+                        if (
+                            substr($this->contents, $this->index, $type['length']) === $type['name']
+                            && Preg::isMatch($type['pattern'], $this->contents, $match, 0, $this->index - 1)
+                        ) {
+                            return $clean . $match[0];
+                        }
                     }
                 }
 
diff --git a/src/PhpFileParser.php b/src/PhpFileParser.php
index 73ed349..49eeb8d 100644
--- a/src/PhpFileParser.php
+++ b/src/PhpFileParser.php
@@ -20,6 +20,20 @@
  */
 class PhpFileParser
 {
+    /**
+     * Regex fragment matching a named extension declaration (proposed "Extension Methods" RFC)
+     *
+     * Captures the declared name (extname) and the raw target token after "on" (exttarget). The
+     * target is captured exactly as written in the source — possibly a relative name or an alias
+     * from a "use" import, unresolved — in anticipation of the target-hint optimization described
+     * in the RFC's Future Scope. It is not used yet and is not part of the class map output.
+     *
+     * Must remain embeddable in a case-insensitive, whitespace-extended ("ix") pattern.
+     *
+     * @internal
+     */
+    public const EXTENSION_REGEX = '\b(?<![\\\\$:>])(?P<ext>extension) \s++ (?P<extname>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+) \s++ on \s++ (?P<exttarget>\\\\?[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\s*+\\\\\s*+[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)*+) \s*+ \$[a-zA-Z_\x7f-\xff]';
+
     /**
      * Extract the classes in the given file
      *
@@ -59,7 +73,7 @@ public static function findClasses(string $path): array
         }
 
         // return early if there is no chance of matching anything in this file
-        Preg::matchAllStrictGroups('{\b(?:class|interface|trait'.$extraTypes.')\s}i', $contents, $matches);
+        Preg::matchAllStrictGroups('{\b(?:class|interface|trait|extension'.$extraTypes.')\s}i', $contents, $matches);
         if ([] === $matches[0]) {
             return [];
         }
@@ -71,6 +85,7 @@ public static function findClasses(string $path): array
         Preg::matchAll('{
             (?:
                  \b(?<![\\\\$:>])(?P<type>class|interface|trait'.$extraTypes.') \s++ (?P<name>[a-zA-Z_\x7f-\xff:][a-zA-Z0-9_\x7f-\xff:\-]*+)
+               | '.self::EXTENSION_REGEX.'
                | \b(?<![\\\\$:>])(?P<ns>namespace) (?P<nsname>\s++[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\s*+\\\\\s*+[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)*+)? \s*+ [\{;]
             )
         }ix', $contents, $matches);
@@ -81,6 +96,16 @@ public static function findClasses(string $path): array
         for ($i = 0, $len = \count($matches['type']); $i < $len; ++$i) {
             if (isset($matches['ns'][$i]) && $matches['ns'][$i] !== '') {
                 $namespace = str_replace([' ', "\t", "\r", "\n"], '', (string) $matches['nsname'][$i]) . '\\';
+            } elseif (isset($matches['ext'][$i]) && $matches['ext'][$i] !== '') {
+                // named extension declarations (proposed "Extension Methods" RFC) declare the identifier
+                // before "on"; the target type after "on" and the receiver variable are not part of the name,
+                // and the anonymous form ("extension Target $var {") declares no autoloadable symbol at all
+                $name = $matches['extname'][$i];
+                \assert(\is_string($name));
+
+                /** @var class-string */
+                $className = ltrim($namespace . $name, '\\');
+                $classes[] = $className;
             } else {
                 $name = $matches['name'][$i];
                 \assert(\is_string($name));
@@ -131,7 +156,7 @@ private static function getExtraTypes(): string
                 $extraTypesArray = ['enum'];
             }
 
-            PhpFileCleaner::setTypeConfig(array_merge(['class', 'interface', 'trait'], $extraTypesArray));
+            PhpFileCleaner::setTypeConfig(array_merge(['class', 'interface', 'trait', 'extension'], $extraTypesArray));
         }
 
         return $extraTypes;
diff --git a/tests/ClassMapGeneratorTest.php b/tests/ClassMapGeneratorTest.php
index 5c0f7e6..7989128 100644
--- a/tests/ClassMapGeneratorTest.php
+++ b/tests/ClassMapGeneratorTest.php
@@ -117,6 +117,20 @@ public function getTestCreateMapTests(): array
             'Dummy\Test\AnonClassHolder' => __DIR__ . '/Fixtures/php7.0/anonclass.php',
         ]];
 
+        // proposed "Extension Methods" RFC, recognized on any PHP version as parsing is purely text-based
+        $data[] = [__DIR__ . '/Fixtures/extensions', [
+            'DomTraversal' => __DIR__ . '/Fixtures/extensions/ExtensionGlobal.php',
+            'Acme\DomKit\DomTraversal' => __DIR__ . '/Fixtures/extensions/ExtensionNamespaced.php',
+            'Acme\Scalars\StringHelpers' => __DIR__ . '/Fixtures/extensions/ExtensionScalarTargets.php',
+            'Acme\Scalars\ArrayHelpers' => __DIR__ . '/Fixtures/extensions/ExtensionScalarTargets.php',
+            'Acme\Aliased\WidgetShortcuts' => __DIR__ . '/Fixtures/extensions/ExtensionAliasedTarget.php',
+            'Acme\Multiline\WidgetHelpers' => __DIR__ . '/Fixtures/extensions/ExtensionMultiline.php',
+            'Acme\Multiline\CaseInsensitive' => __DIR__ . '/Fixtures/extensions/ExtensionMultiline.php',
+            'Acme\Tricky\extension' => __DIR__ . '/Fixtures/extensions/ExtensionFalsePositives.php',
+            'Acme\Mixed\Formatter' => __DIR__ . '/Fixtures/extensions/ExtensionWithClass.php',
+            'Acme\Mixed\FormatterHelpers' => __DIR__ . '/Fixtures/extensions/ExtensionWithClass.php',
+        ]];
+
         if (PHP_VERSION_ID >= 80100) {
             $data[] = [__DIR__ . '/Fixtures/php8.1', [
                 'RolesBasicEnum' => __DIR__ . '/Fixtures/php8.1/enum_basic.php',
@@ -414,6 +428,45 @@ public function testGetRawPSR4Violations(): void
         self::assertSame('UnexpectedNamespace\ClassWithNameSpaceOutsideConfiguredScope', $rawViolations[$classWithNameSpaceOutsideConfiguredScopeFilepath][0]['className']);
     }
 
+    public function testExtensionDeclarationsFollowPsr4Rules(): void
+    {
+        $this->generator->scanPaths(__DIR__ . '/Fixtures/psr4Extensions', null, 'psr-4', 'ExpectedNamespace\\');
+        $classMap = $this->generator->getClassMap();
+
+        // a named extension declaration matching the file path is accepted as the file's expected symbol
+        self::assertArrayHasKey('ExpectedNamespace\\DomTraversal', $classMap->getMap());
+
+        self::assertSame(
+            [
+                'Class ExpectedNamespace\SomeOtherName located in ./tests/Fixtures/psr4Extensions/ExtensionWithIncorrectName.php does not comply with psr-4 autoloading standard (rule: ExpectedNamespace\ => ./tests/Fixtures/psr4Extensions). Skipping.',
+            ],
+            $classMap->getPsrViolations()
+        );
+    }
+
+    /**
+     * Extension names live in the same name=>file map as class-like symbols, so
+     * duplicates produce the usual ambiguous class warnings
+     */
+    public function testExtensionNamesShareNamespaceWithClasses(): void
+    {
+        $tempDir = self::getUniqueTmpDirectory();
+        mkdir($tempDir.'/other');
+
+        file_put_contents($tempDir . '/A.php', "<?php\nclass A {}");
+        file_put_contents($tempDir . '/other/A.php', "<?php\nextension A on \\ArrayObject \$a {}");
+
+        $this->generator->scanPaths($tempDir);
+        $classMap = $this->generator->getClassMap();
+
+        self::assertCount(1, $classMap->getAmbiguousClasses());
+        self::assertArrayHasKey('A', $classMap->getAmbiguousClasses());
+        self::assertCount(1, $classMap->getAmbiguousClasses()['A']);
+
+        $fs = new Filesystem();
+        $fs->remove($tempDir);
+    }
+
     public function testCreateMapWithDirectoryExcluded(): void
     {
         $expected = [
diff --git a/tests/Fixtures/extensions/ExtensionAliasedTarget.php b/tests/Fixtures/extensions/ExtensionAliasedTarget.php
new file mode 100644
index 0000000..523e678
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionAliasedTarget.php
@@ -0,0 +1,9 @@
+<?php
+
+namespace Acme\Aliased;
+
+use Acme\Widgets\Widget as W;
+
+extension WidgetShortcuts on W $w
+{
+}
diff --git a/tests/Fixtures/extensions/ExtensionAnonymous.php b/tests/Fixtures/extensions/ExtensionAnonymous.php
new file mode 100644
index 0000000..6413d02
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionAnonymous.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace Acme\Anon;
+
+// anonymous extensions have no name and are not autoloadable, they must not end up in the class map
+
+extension \DateTimeImmutable $date
+{
+    public function tomorrow(): \DateTimeImmutable
+    {
+        return $date->modify('+1 day');
+    }
+}
+
+extension string $s
+{
+    public function shout(): string
+    {
+        return \strtoupper($s);
+    }
+}
diff --git a/tests/Fixtures/extensions/ExtensionFalsePositives.php b/tests/Fixtures/extensions/ExtensionFalsePositives.php
new file mode 100644
index 0000000..2dc177b
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionFalsePositives.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Acme\Tricky;
+
+// "extension" is a valid symbol name in PHP today, and the RFC's import
+// statement ("use extension ...") declares nothing; none of the usages below
+// may produce a class map entry except the class actually named "extension"
+
+use extension Acme\DomKit\DomTraversal;
+
+class extension extends \ArrayObject
+{
+    public function extension(string $x): string
+    {
+        return \pathinfo($x, \PATHINFO_EXTENSION);
+    }
+}
+
+function extension(string $s): string
+{
+    $obj = new extension();
+    $obj->extension($s);
+
+    return 'extension Foo on Bar $b { }';
+}
diff --git a/tests/Fixtures/extensions/ExtensionGlobal.php b/tests/Fixtures/extensions/ExtensionGlobal.php
new file mode 100644
index 0000000..b856014
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionGlobal.php
@@ -0,0 +1,9 @@
+<?php
+
+extension DomTraversal on \DOMElement $el
+{
+    public function firstByClass(string $class): ?\DOMElement
+    {
+        return null;
+    }
+}
diff --git a/tests/Fixtures/extensions/ExtensionMultiline.php b/tests/Fixtures/extensions/ExtensionMultiline.php
new file mode 100644
index 0000000..9a77cec
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionMultiline.php
@@ -0,0 +1,17 @@
+<?php
+
+namespace Acme\Multiline;
+
+extension
+    // the declaration may be spread over several lines
+    WidgetHelpers
+    /* with comments between any two tokens */
+    on
+    \Acme\Widgets\Widget
+    $widget
+{
+}
+
+EXTENSION CaseInsensitive ON Widgets\Widget $w
+{
+}
diff --git a/tests/Fixtures/extensions/ExtensionNamespaced.php b/tests/Fixtures/extensions/ExtensionNamespaced.php
new file mode 100644
index 0000000..328c3e1
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionNamespaced.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Acme\DomKit;
+
+extension DomTraversal on \DOMElement $el
+{
+    public function firstByClass(string $class): ?\DOMElement
+    {
+        return null;
+    }
+}
diff --git a/tests/Fixtures/extensions/ExtensionScalarTargets.php b/tests/Fixtures/extensions/ExtensionScalarTargets.php
new file mode 100644
index 0000000..470f9a1
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionScalarTargets.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace Acme\Scalars;
+
+extension StringHelpers on string $s
+{
+    public function isBlank(): bool
+    {
+        return \trim($s) === '';
+    }
+}
+
+extension ArrayHelpers on array $items
+{
+    public function isEmpty(): bool
+    {
+        return $items === [];
+    }
+}
diff --git a/tests/Fixtures/extensions/ExtensionWithClass.php b/tests/Fixtures/extensions/ExtensionWithClass.php
new file mode 100644
index 0000000..9cca66f
--- /dev/null
+++ b/tests/Fixtures/extensions/ExtensionWithClass.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Acme\Mixed;
+
+class Formatter
+{
+}
+
+extension FormatterHelpers on Formatter $formatter
+{
+}
diff --git a/tests/Fixtures/psr4Extensions/DomTraversal.php b/tests/Fixtures/psr4Extensions/DomTraversal.php
new file mode 100644
index 0000000..3c10fc5
--- /dev/null
+++ b/tests/Fixtures/psr4Extensions/DomTraversal.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace ExpectedNamespace;
+
+extension DomTraversal on \DOMElement $el
+{
+    public function firstByClass(string $class): ?\DOMElement
+    {
+        return null;
+    }
+}
diff --git a/tests/Fixtures/psr4Extensions/ExtensionWithIncorrectName.php b/tests/Fixtures/psr4Extensions/ExtensionWithIncorrectName.php
new file mode 100644
index 0000000..6a5e2c0
--- /dev/null
+++ b/tests/Fixtures/psr4Extensions/ExtensionWithIncorrectName.php
@@ -0,0 +1,7 @@
+<?php
+
+namespace ExpectedNamespace;
+
+extension SomeOtherName on string $s
+{
+}
diff --git a/tests/PhpFileParserTest.php b/tests/PhpFileParserTest.php
index 6d9f113..8e97970 100644
--- a/tests/PhpFileParserTest.php
+++ b/tests/PhpFileParserTest.php
@@ -18,6 +18,7 @@
 
 namespace Composer\ClassMapGenerator;
 
+use Composer\Pcre\Preg;
 use PHPUnit\Framework\TestCase;
 
 class PhpFileParserTest extends TestCase
@@ -28,4 +29,62 @@ public function testFindClassesThrowsWhenFileDoesNotExist(): void
         self::expectExceptionMessage('does not exist');
         PhpFileParser::findClasses(__DIR__ . '/no-file');
     }
+
+    /**
+     * The raw target token after "on" is captured as written (relative names and import
+     * aliases stay unresolved), in anticipation of the target-hint optimization from the
+     * Extension Methods RFC's Future Scope. It is not part of the class map output.
+     *
+     * @dataProvider extensionTargetProvider
+     * @param array<string, string> $expected extension name => raw target token
+     */
+    public function testExtensionTargetIsCapturedRaw(string $file, array $expected): void
+    {
+        PhpFileCleaner::setTypeConfig(['class', 'interface', 'trait', 'extension', 'enum']);
+
+        $contents = php_strip_whitespace($file);
+        // maxMatches > 1 disables the cleaner's single-match early return so the whole file is cleaned
+        $cleaner = new PhpFileCleaner($contents, PHP_INT_MAX);
+        $contents = $cleaner->clean();
+
+        Preg::matchAllStrictGroups('{'.PhpFileParser::EXTENSION_REGEX.'}ix', $contents, $matches);
+
+        $actual = [];
+        foreach ($matches['extname'] as $i => $name) {
+            $actual[$name] = $matches['exttarget'][$i];
+        }
+
+        self::assertSame($expected, $actual);
+    }
+
+    /**
+     * @return array<string, array{string, array<string, string>}>
+     */
+    public static function extensionTargetProvider(): array
+    {
+        return [
+            'fully-qualified target' => [__DIR__ . '/Fixtures/extensions/ExtensionGlobal.php', [
+                'DomTraversal' => '\DOMElement',
+            ]],
+            'fully-qualified target in namespace' => [__DIR__ . '/Fixtures/extensions/ExtensionNamespaced.php', [
+                'DomTraversal' => '\DOMElement',
+            ]],
+            'scalar and array targets' => [__DIR__ . '/Fixtures/extensions/ExtensionScalarTargets.php', [
+                'StringHelpers' => 'string',
+                'ArrayHelpers' => 'array',
+            ]],
+            'relative qualified target stays unresolved' => [__DIR__ . '/Fixtures/extensions/ExtensionMultiline.php', [
+                'WidgetHelpers' => '\Acme\Widgets\Widget',
+                'CaseInsensitive' => 'Widgets\Widget',
+            ]],
+            'aliased target stays unresolved' => [__DIR__ . '/Fixtures/extensions/ExtensionAliasedTarget.php', [
+                'WidgetShortcuts' => 'W',
+            ]],
+            'unqualified target' => [__DIR__ . '/Fixtures/extensions/ExtensionWithClass.php', [
+                'FormatterHelpers' => 'Formatter',
+            ]],
+            'anonymous forms capture nothing' => [__DIR__ . '/Fixtures/extensions/ExtensionAnonymous.php', []],
+            'false positives capture nothing' => [__DIR__ . '/Fixtures/extensions/ExtensionFalsePositives.php', []],
+        ];
+    }
 }