diff --git a/tests/SignalExitCodeTest.php b/tests/SignalExitCodeTest.php
new file mode 100644
index 0000000..aa5d603
--- /dev/null
+++ b/tests/SignalExitCodeTest.php
@@ -0,0 +1,132 @@
+<?php
+
+/*
+ * This file is part of composer/xdebug-handler.
+ *
+ * (c) Composer <https://github.com/composer>
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Composer\XdebugHandler\Tests;
+
+use PHPUnit\Framework\TestCase;
+
+class SignalExitCodeTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+            self::markTestSkipped('Signal handling tests are not applicable on Windows');
+        }
+
+        if (!function_exists('posix_kill') || !function_exists('posix_getpid')) {
+            self::markTestSkipped('posix extension is required');
+        }
+    }
+
+    public function testProcCloseReturnsRawSignalNumberForSigterm(): void
+    {
+        $script = 'posix_kill(posix_getpid(), 15);'; // SIGTERM
+        $process = proc_open(
+            [PHP_BINARY, '-r', $script],
+            [],
+            $pipes
+        );
+
+        self::assertIsResource($process);
+
+        // proc_close returns the raw signal number (15), not 128+15
+        $exitCode = proc_close($process);
+        self::assertSame(15, $exitCode);
+    }
+
+    public function testProcCloseReturnsRawSignalNumberForSegfault(): void
+    {
+        $script = 'posix_kill(posix_getpid(), 11);'; // SIGSEGV
+        $process = proc_open(
+            [PHP_BINARY, '-r', $script],
+            [],
+            $pipes
+        );
+
+        self::assertIsResource($process);
+
+        $exitCode = proc_close($process);
+        self::assertSame(11, $exitCode);
+    }
+
+    public function testProcGetStatusDetectsSignalDeath(): void
+    {
+        $script = 'posix_kill(posix_getpid(), 15);'; // SIGTERM
+        $process = proc_open(
+            [PHP_BINARY, '-r', $script],
+            [],
+            $pipes
+        );
+
+        self::assertIsResource($process);
+
+        $status = proc_get_status($process);
+        while ($status['running']) {
+            usleep(10_000);
+            $status = proc_get_status($process);
+        }
+
+        self::assertTrue($status['signaled']);
+        self::assertSame(15, $status['termsig']);
+        self::assertSame(143, 128 + $status['termsig']);
+
+        proc_close($process);
+    }
+
+    public function testProcGetStatusDetectsSegfault(): void
+    {
+        $script = 'posix_kill(posix_getpid(), 11);'; // SIGSEGV
+        $process = proc_open(
+            [PHP_BINARY, '-r', $script],
+            [],
+            $pipes
+        );
+
+        self::assertIsResource($process);
+
+        $status = proc_get_status($process);
+        while ($status['running']) {
+            usleep(10_000);
+            $status = proc_get_status($process);
+        }
+
+        self::assertTrue($status['signaled']);
+        self::assertSame(11, $status['termsig']);
+        self::assertSame(139, 128 + $status['termsig']);
+
+        proc_close($process);
+    }
+
+    public function testNormalExitCodeIsPreserved(): void
+    {
+        $script = 'exit(42);';
+        $process = proc_open(
+            [PHP_BINARY, '-r', $script],
+            [],
+            $pipes
+        );
+
+        self::assertIsResource($process);
+
+        $status = proc_get_status($process);
+        while ($status['running']) {
+            usleep(10_000);
+            $status = proc_get_status($process);
+        }
+
+        self::assertFalse($status['signaled']);
+        self::assertSame(42, $status['exitcode']);
+
+        proc_close($process);
+    }
+}