diff --git a/.github/workflows/e2e-tests.yml b/.github/workflows/e2e-tests.yml
index df2ca12db6..11504b4448 100644
--- a/.github/workflows/e2e-tests.yml
+++ b/.github/workflows/e2e-tests.yml
@@ -128,6 +128,10 @@ jobs:
               cd e2e/bug-14514
               composer install
               ../../bin/phpstan analyze bug-14515.php
+          - script: |
+              cd e2e/bug-14988
+              composer install
+              ../../bin/phpstan analyse
           - script: |
               cd e2e/bug-14724
               composer install
diff --git a/e2e/bug-14988/.gitignore b/e2e/bug-14988/.gitignore
new file mode 100644
index 0000000000..61ead86667
--- /dev/null
+++ b/e2e/bug-14988/.gitignore
@@ -0,0 +1 @@
+/vendor
diff --git a/e2e/bug-14988/classes/gadget.php b/e2e/bug-14988/classes/gadget.php
new file mode 100644
index 0000000000..5ff2deec60
--- /dev/null
+++ b/e2e/bug-14988/classes/gadget.php
@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace Redeclare\Builder;
+
+class gadget
+{
+
+	public int $size = 3;
+
+}
diff --git a/e2e/bug-14988/composer.json b/e2e/bug-14988/composer.json
new file mode 100644
index 0000000000..18fc06991e
--- /dev/null
+++ b/e2e/bug-14988/composer.json
@@ -0,0 +1,7 @@
+{
+	"autoload": {
+		"files": [
+			"pkg/bootstrap.php"
+		]
+	}
+}
diff --git a/e2e/bug-14988/composer.lock b/e2e/bug-14988/composer.lock
new file mode 100644
index 0000000000..ba8d41762c
--- /dev/null
+++ b/e2e/bug-14988/composer.lock
@@ -0,0 +1,18 @@
+{
+    "_readme": [
+        "This file locks the dependencies of your project to a known state",
+        "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
+        "This file is @generated automatically"
+    ],
+    "content-hash": "d751713988987e9331980363e24189ce",
+    "packages": [],
+    "packages-dev": [],
+    "aliases": [],
+    "minimum-stability": "stable",
+    "stability-flags": {},
+    "prefer-stable": false,
+    "prefer-lowest": false,
+    "platform": {},
+    "platform-dev": {},
+    "plugin-api-version": "2.9.0"
+}
diff --git a/e2e/bug-14988/custom-autoloader.php b/e2e/bug-14988/custom-autoloader.php
new file mode 100644
index 0000000000..808df847e5
--- /dev/null
+++ b/e2e/bug-14988/custom-autoloader.php
@@ -0,0 +1,43 @@
+<?php declare(strict_types=1);
+
+namespace Redeclare;
+
+use function is_file;
+use function spl_autoload_register;
+use function strlen;
+use function strncmp;
+use function substr;
+
+/**
+ * Stands in for the real-world autoloader that reproduces phpstan/phpstan#14988: PHP_CodeSniffer's
+ * own autoloader, loaded via bootstrapFiles for a package that ships no Composer autoload metadata.
+ * It shares the three properties that make it fatal:
+ *
+ *  1. It is registered as a string callable ("Class::method"), which spl_autoload_functions()
+ *     normalises to ['Class', 'method'] - an array whose first element is a string, not a
+ *     ClassLoader object - so it survives bin/phpstan's Composer ClassLoader exclusion.
+ *  2. It is catch-all: it resolves names outside its own namespace.
+ *  3. It uses a plain include (not include_once), so a file already loaded is executed again.
+ */
+final class Autoloader
+{
+
+	public static function load(string $class): void
+	{
+		if (strncmp($class, 'Redeclare\\Builder\\', 18) !== 0) {
+			return;
+		}
+
+		$name = substr($class, strlen('Redeclare\\Builder\\'));
+		foreach (['/classes/', '/pkg/'] as $dir) {
+			$file = __DIR__ . $dir . $name . '.php';
+			if (is_file($file)) {
+				include $file;
+				return;
+			}
+		}
+	}
+
+}
+
+spl_autoload_register('Redeclare\\Autoloader::load', true, true);
diff --git a/e2e/bug-14988/phpstan.neon b/e2e/bug-14988/phpstan.neon
new file mode 100644
index 0000000000..6310a322f3
--- /dev/null
+++ b/e2e/bug-14988/phpstan.neon
@@ -0,0 +1,12 @@
+parameters:
+	level: 0
+	paths:
+		- src
+	bootstrapFiles:
+		- custom-autoloader.php
+	ignoreErrors:
+		# The probe deliberately references a name that is a function, not a class. The point of
+		# this fixture is that analysis completes instead of fatally re-including the function file.
+		-
+			message: '#^Class Redeclare\\Builder\\thing not found\.$#'
+			path: src/probe.php
diff --git a/e2e/bug-14988/pkg/bootstrap.php b/e2e/bug-14988/pkg/bootstrap.php
new file mode 100644
index 0000000000..e901ff81f2
--- /dev/null
+++ b/e2e/bug-14988/pkg/bootstrap.php
@@ -0,0 +1,9 @@
+<?php declare(strict_types=1);
+
+if (!function_exists('Redeclare\Builder\thing')) {
+	require_once __DIR__ . '/thing.php';
+}
+
+if (!function_exists('Redeclare\Builder\gadget')) {
+	require_once __DIR__ . '/gadget.php';
+}
diff --git a/e2e/bug-14988/pkg/gadget.php b/e2e/bug-14988/pkg/gadget.php
new file mode 100644
index 0000000000..0765019e53
--- /dev/null
+++ b/e2e/bug-14988/pkg/gadget.php
@@ -0,0 +1,8 @@
+<?php declare(strict_types=1);
+
+namespace Redeclare\Builder;
+
+function gadget(): int
+{
+	return 1;
+}
diff --git a/e2e/bug-14988/pkg/thing.php b/e2e/bug-14988/pkg/thing.php
new file mode 100644
index 0000000000..33ab9ccc40
--- /dev/null
+++ b/e2e/bug-14988/pkg/thing.php
@@ -0,0 +1,8 @@
+<?php declare(strict_types=1);
+
+namespace Redeclare\Builder;
+
+function thing(): string
+{
+	return 'thing';
+}
diff --git a/e2e/bug-14988/src/probe.php b/e2e/bug-14988/src/probe.php
new file mode 100644
index 0000000000..34d0ffe5f7
--- /dev/null
+++ b/e2e/bug-14988/src/probe.php
@@ -0,0 +1,24 @@
+<?php declare(strict_types=1);
+
+namespace App;
+
+use Redeclare\Builder\gadget;
+
+class Probe
+{
+
+	// Probing a name that is only a function forces a hasClass() lookup, which used to re-run the
+	// custom autoloader and fatally re-include the already-loaded function file.
+	public function isThing(object $o): bool
+	{
+		return $o instanceof \Redeclare\Builder\thing;
+	}
+
+	// A class and a function may share a name in PHP. Even though the function is already loaded,
+	// the class defined in a separate file must still resolve (via the later source locators).
+	public function makeGadget(): int
+	{
+		return (new gadget())->size;
+	}
+
+}
diff --git a/src/Reflection/BetterReflection/SourceLocator/AutoloadFunctionsSourceLocator.php b/src/Reflection/BetterReflection/SourceLocator/AutoloadFunctionsSourceLocator.php
index 3da44c64bf..750c0a175b 100644
--- a/src/Reflection/BetterReflection/SourceLocator/AutoloadFunctionsSourceLocator.php
+++ b/src/Reflection/BetterReflection/SourceLocator/AutoloadFunctionsSourceLocator.php
@@ -9,6 +9,7 @@
 use PHPStan\BetterReflection\Reflector\Reflector;
 use PHPStan\BetterReflection\SourceLocator\Type\SourceLocator;
 use function class_exists;
+use function function_exists;
 use function interface_exists;
 use function PHPStan\autoloadFunctions;
 use function trait_exists;
@@ -35,6 +36,18 @@ public function locateIdentifier(Reflector $reflector, Identifier $identifier):
 			return null;
 		}
 
+		// If the name is already a defined function, this locator must not run the bootstrap
+		// autoloaders for it: a catch-all autoloader (e.g. PHP_CodeSniffer's, which falls back to
+		// Composer's findFile()) would resolve the name to the function's own file and plain-include
+		// it a second time - it was loaded once already, e.g. by a package that ships one function
+		// per PSR-4 path and requires it from its bootstrap - fatally redeclaring the function.
+		// Returning null only declines this locator; a class and a function may share a name in PHP,
+		// and a class that genuinely exists under this name in another file is still located by the
+		// later source locators in the chain. See https://github.com/phpstan/phpstan/issues/14988
+		if (function_exists($className)) {
+			return null;
+		}
+
 		$autoloadFunctions = autoloadFunctions();
 		foreach ($autoloadFunctions as $autoloadFunction) {
 			$autoloadFunction($className);