diff --git a/src/Installing/UnixInstall.php b/src/Installing/UnixInstall.php
index a58c14db..0084655b 100644
--- a/src/Installing/UnixInstall.php
+++ b/src/Installing/UnixInstall.php
@@ -5,6 +5,7 @@
 namespace Php\Pie\Installing;
 
 use Composer\IO\IOInterface;
+use Composer\Util\Platform as ComposerPlatform;
 use Php\Pie\Downloading\DownloadedPackage;
 use Php\Pie\Downloading\DownloadUrlMethod;
 use Php\Pie\File\BinaryFile;
@@ -37,7 +38,14 @@ public function __invoke(
         IOInterface $io,
         bool $attemptToSetupIniFile,
     ): BinaryFile {
-        $targetExtensionPath = $targetPlatform->phpBinaryPath->extensionPath();
+        $env         = [];
+        $installRoot = (string) ComposerPlatform::getEnv('INSTALL_ROOT');
+        if ($installRoot !== '') {
+            $io->write(sprintf('<info>Using INSTALL_ROOT=%s</info>', $installRoot));
+            $env['INSTALL_ROOT'] = $installRoot;
+        }
+
+        $targetExtensionPath = $targetPlatform->phpBinaryPath->extensionPath($installRoot);
 
         $sharedObjectName             = $downloadedPackage->package->extensionName()->name() . '.so';
         $expectedSharedObjectLocation = sprintf(
@@ -93,6 +101,7 @@ public function __invoke(
                 $installCommand,
                 $downloadedPackage->extractedSourcePath,
                 self::MAKE_INSTALL_TIMEOUT_SECS,
+                env: $env,
             );
 
             $io->write($makeInstallOutput, verbosity: IOInterface::VERY_VERBOSE);
diff --git a/src/Platform/TargetPhp/PhpBinaryPath.php b/src/Platform/TargetPhp/PhpBinaryPath.php
index 32108921..f0f231c9 100644
--- a/src/Platform/TargetPhp/PhpBinaryPath.php
+++ b/src/Platform/TargetPhp/PhpBinaryPath.php
@@ -30,9 +30,11 @@
 use function in_array;
 use function is_dir;
 use function is_executable;
+use function ltrim;
 use function mkdir;
 use function preg_match;
 use function preg_replace;
+use function rtrim;
 use function sprintf;
 use function strtolower;
 use function trim;
@@ -103,7 +105,7 @@ public function debugMode(): DebugBuild
     }
 
     /** @return non-empty-string */
-    public function extensionPath(): string
+    public function extensionPath(string|null $prefixInstallRoot = null): string
     {
         $phpinfo = $this->phpinfo();
 
@@ -116,6 +118,10 @@ public function extensionPath(): string
             $extensionPath = trim($matches[1]);
             assert($extensionPath !== '');
 
+            if (self::operatingSystem() !== OperatingSystem::Windows && $prefixInstallRoot !== null && $prefixInstallRoot !== '') {
+                $extensionPath = rtrim($prefixInstallRoot, DIRECTORY_SEPARATOR) . DIRECTORY_SEPARATOR . ltrim($extensionPath, DIRECTORY_SEPARATOR);
+            }
+
             if (file_exists($extensionPath) && is_dir($extensionPath)) {
                 return $extensionPath;
             }
diff --git a/src/Util/Process.php b/src/Util/Process.php
index 329ce5f2..8e32df16 100644
--- a/src/Util/Process.php
+++ b/src/Util/Process.php
@@ -30,6 +30,7 @@ private function __construct()
      *
      * @param list<string>                                                         $command
      * @param callable(SymfonyProcess::ERR|SymfonyProcess::OUT, string): void|null $outputCallback
+     * @param array<string, scalar>|null                                           $env
      *
      * @throws ProcessFailedException
      */
@@ -38,8 +39,9 @@ public static function run(
         string|null $workingDirectory = null,
         int|null $timeout = self::NO_TIMEOUT,
         callable|null $outputCallback = null,
+        array|null $env = null,
     ): string {
-        return trim((new SymfonyProcess($command, $workingDirectory, timeout: $timeout))
+        return trim((new SymfonyProcess($command, $workingDirectory, $env, timeout: $timeout))
             ->mustRun($outputCallback)
             ->getOutput());
     }
diff --git a/test/integration/Installing/UnixInstallTest.php b/test/integration/Installing/UnixInstallTest.php
index f80e21d8..820146ed 100644
--- a/test/integration/Installing/UnixInstallTest.php
+++ b/test/integration/Installing/UnixInstallTest.php
@@ -6,6 +6,7 @@
 
 use Composer\IO\BufferIO;
 use Composer\Package\CompletePackageInterface;
+use Composer\Util\Filesystem;
 use Composer\Util\Platform;
 use Php\Pie\Building\UnixBuild;
 use Php\Pie\DependencyResolver\Package;
@@ -30,11 +31,13 @@
 use function array_unshift;
 use function assert;
 use function file_exists;
+use function getenv;
 use function is_executable;
 use function is_writable;
 use function mkdir;
+use function putenv;
 use function rename;
-use function unlink;
+use function uniqid;
 
 use const DIRECTORY_SEPARATOR;
 
@@ -79,6 +82,10 @@ public static function phpPathProvider(): array
     #[DataProvider('phpPathProvider')]
     public function testUnixInstallCanInstallExtensionBuiltFromSource(string $phpConfig): void
     {
+        $installRoot    = '/tmp/' . uniqid('pie-test-install-root-', true);
+        $oldInstallRoot = getenv('INSTALL_ROOT');
+        putenv('INSTALL_ROOT=' . $installRoot);
+
         assert($phpConfig !== '');
         if (Platform::isWindows()) {
             self::markTestSkipped('Unix build test cannot be run on Windows');
@@ -86,7 +93,7 @@ public function testUnixInstallCanInstallExtensionBuiltFromSource(string $phpCon
 
         $output         = new BufferIO();
         $targetPlatform = TargetPlatform::fromPhpBinaryPath(PhpBinaryPath::fromPhpConfigExecutable($phpConfig), null);
-        $extensionPath  = $targetPlatform->phpBinaryPath->extensionPath();
+        $extensionPath  = $targetPlatform->phpBinaryPath->extensionPath($installRoot);
 
         $composerPackage = $this->createMock(CompletePackageInterface::class);
         $composerPackage
@@ -136,11 +143,17 @@ public function testUnixInstallCanInstallExtensionBuiltFromSource(string $phpCon
         (new Process($rmCommand))->mustRun();
         (new Process(['make', 'clean'], $downloadedPackage->extractedSourcePath))->mustRun();
         (new Process(['phpize', '--clean'], $downloadedPackage->extractedSourcePath))->mustRun();
+        (new Filesystem())->remove($installRoot);
+        putenv('INSTALL_ROOT=' . $oldInstallRoot);
     }
 
     #[DataProvider('phpPathProvider')]
     public function testUnixInstallCanInstallPrePackagedBinary(string $phpConfig): void
     {
+        $installRoot    = '/tmp/' . uniqid('pie-test-install-root-', true);
+        $oldInstallRoot = getenv('INSTALL_ROOT');
+        putenv('INSTALL_ROOT=' . $installRoot);
+
         assert($phpConfig !== '');
         if (Platform::isWindows()) {
             self::markTestSkipped('Unix build test cannot be run on Windows');
@@ -148,7 +161,8 @@ public function testUnixInstallCanInstallPrePackagedBinary(string $phpConfig): v
 
         $output         = new BufferIO();
         $targetPlatform = TargetPlatform::fromPhpBinaryPath(PhpBinaryPath::fromPhpConfigExecutable($phpConfig), null);
-        $extensionPath  = $targetPlatform->phpBinaryPath->extensionPath();
+        $extensionPath  = $installRoot . $targetPlatform->phpBinaryPath->extensionPath();
+        mkdir($extensionPath, 0777, true);
 
         // First build it (otherwise the test assets would need to have a binary for every test platform...)
         $composerPackage = $this->createMock(CompletePackageInterface::class);
@@ -220,6 +234,8 @@ public function testUnixInstallCanInstallPrePackagedBinary(string $phpConfig): v
         }
 
         (new Process($rmCommand))->mustRun();
-        unlink($prebuiltBinaryFile->filePath);
+        (new Filesystem())->remove($prebuiltBinaryFile->filePath);
+        (new Filesystem())->remove($installRoot);
+        putenv('INSTALL_ROOT=' . $oldInstallRoot);
     }
 }
diff --git a/test/unit/Platform/TargetPhp/PhpBinaryPathTest.php b/test/unit/Platform/TargetPhp/PhpBinaryPathTest.php
index 321ac7d8..eeeda371 100644
--- a/test/unit/Platform/TargetPhp/PhpBinaryPathTest.php
+++ b/test/unit/Platform/TargetPhp/PhpBinaryPathTest.php
@@ -276,6 +276,22 @@ public function testExtensionPathOnLinuxThatAlreadyExists(): void
         );
     }
 
+    #[RequiresOperatingSystemFamily('Linux')]
+    public function testExtensionPathWithInstallRootPrefixOnLinuxThatAlreadyExists(): void
+    {
+        $installRoot = '/tmp/' . uniqid('pie-test-install-root-existing-', true);
+        $phpBinary   = PhpBinaryPath::fromCurrentProcess();
+
+        $expectedExtensionDir = $installRoot . ini_get('extension_dir');
+        mkdir($expectedExtensionDir, 0777, true);
+        self::assertDirectoryExists($expectedExtensionDir);
+
+        self::assertSame(
+            $expectedExtensionDir,
+            $phpBinary->extensionPath($installRoot),
+        );
+    }
+
     #[RequiresOperatingSystemFamily('Windows')]
     public function testExtensionPathOnWindows(): void
     {
@@ -336,6 +352,19 @@ public function testExtensionPathIsImplicitlyCreated(): void
         self::assertDirectoryExists($configuredExtensionPath);
     }
 
+    #[RequiresOperatingSystemFamily('Linux')]
+    public function testExtensionPathWithInstallRootPrefixIsImplicitlyCreated(): void
+    {
+        $installRoot = '/tmp/' . uniqid('pie-test-install-root-not-existing-', true);
+        $phpBinary   = PhpBinaryPath::fromCurrentProcess();
+
+        $expectedExtensionDir = $installRoot . ini_get('extension_dir');
+        self::assertDirectoryDoesNotExist($expectedExtensionDir);
+
+        self::assertSame($expectedExtensionDir, $phpBinary->extensionPath($installRoot));
+        self::assertDirectoryExists($expectedExtensionDir);
+    }
+
     /** @return array<string, array{0: string}> */
     public static function phpPathProvider(): array
     {