diff --git a/phpunit/src/Installer/PhpBuildConfigurationTest.php b/phpunit/src/Installer/PhpBuildConfigurationTest.php
index 5498a20d..d5eca6df 100644
--- a/phpunit/src/Installer/PhpBuildConfigurationTest.php
+++ b/phpunit/src/Installer/PhpBuildConfigurationTest.php
@@ -57,7 +57,6 @@ final class PhpBuildConfigurationTest extends TestCase
             ['--includedir=/usr/include', '--disable-all', '--with-zlib=/usr'],
             $parsed
         );
-        self::assertContains('--includedir=/usr/include', $options);
         self::assertContains('--disable-all', $options);
         self::assertContains('--with-zlib=/usr', $options);
         self::assertNotContains('CFLAGS=-g', $options);
@@ -78,6 +77,50 @@ final class PhpBuildConfigurationTest extends TestCase
         self::assertContains('--enable-cli', $options);
     }
 
+    public function testDeriveKeepsInstallationDirectoriesInsideThePrefix(): void
+    {
+        // A Debian/Ubuntu build records absolute installation directories; reusing
+        // them makes `make install` write to /usr/share/man and /usr/include, which
+        // an unprivileged user cannot do.
+        $options = PhpBuildConfiguration::derive(
+            "'--prefix=/usr' '--exec-prefix=/usr' '--includedir=/usr/include' " .
+            "'--mandir=/usr/share/man' '--infodir=/usr/share/info' '--sysconfdir=/etc' " .
+            "'--localstatedir=/var' '--libdir=\${prefix}/lib/php' " .
+            "'--libexecdir=/usr/lib/x86_64-linux-gnu' '--datadir=\${prefix}/share/php/8.5' " .
+            "'--with-layout=GNU' '--with-openssl'",
+            '/home/test/.typephp'
+        );
+
+        foreach ($options as $option) {
+            self::assertStringNotContainsString('/usr/', $option);
+            self::assertStringNotContainsString('${prefix}', $option);
+        }
+        self::assertSame(['--prefix=/home/test/.typephp'], array_values(array_filter(
+            $options,
+            static fn(string $option): bool => str_starts_with($option, '--prefix=')
+        )));
+        self::assertContains('--with-layout=GNU', $options);
+        self::assertContains('--with-openssl', $options);
+    }
+
+    public function testDeriveDropsProgramNameTransformsAndStaleConfigureCaches(): void
+    {
+        // --program-suffix would install bin/php8.5 instead of bin/php, which the
+        // installer and the platform probes both look for by its plain name.
+        $options = PhpBuildConfiguration::derive(
+            "'--program-suffix=8.5' '--program-prefix=x' '--program-transform-name=s,x,x,' " .
+            "'--config-cache' '--cache-file=/tmp/buildd/nonexistent/config.cache' '--enable-pcntl'",
+            '/home/test/.typephp'
+        );
+
+        self::assertNotContains('--program-suffix=8.5', $options);
+        self::assertNotContains('--program-prefix=x', $options);
+        self::assertNotContains('--program-transform-name=s,x,x,', $options);
+        self::assertNotContains('--config-cache', $options);
+        self::assertNotContains('--cache-file=/tmp/buildd/nonexistent/config.cache', $options);
+        self::assertContains('--enable-pcntl', $options);
+    }
+
     public function testParseShellWordsRejectsIncompleteInput(): void
     {
         $this->expectException(\InvalidArgumentException::class);
diff --git a/src/Installer/LibPhpInstaller.php b/src/Installer/LibPhpInstaller.php
index ad9ed030..e808e1a7 100644
--- a/src/Installer/LibPhpInstaller.php
+++ b/src/Installer/LibPhpInstaller.php
@@ -25,27 +25,49 @@ final class LibPhpInstaller
         }
 
         $this->console->write("The current PHP installation does not provide libphp.so: {$currentPhpDir}");
+
+        $home = getenv('HOME') ?: (string) ($_SERVER['HOME'] ?? '');
+        $defaultPrefix = rtrim($home, '/') . '/.typephp';
+        $defaultVersion = PHP_VERSION;
+
+        // A build left by an earlier run answers every question below, so offer
+        // it before asking any of them. Both defaults are known without asking:
+        // the directory this installer always proposes, and the running PHP.
+        if ($this->offerInstalled($defaultPrefix, $defaultVersion)) {
+            return $this->activate($defaultPrefix);
+        }
+
         if (!$this->console->confirm('Build a private PHP embed library now?', true)) {
             return null;
         }
 
-        $defaultVersion = PHP_VERSION;
         $version = $this->console->ask("PHP version [{$defaultVersion}]: ", $defaultVersion);
         if (!preg_match('/^8\.[45]\.\d+$/', $version)) {
             throw new \RuntimeException('Only stable PHP 8.4.x and 8.5.x versions are supported by the automatic installer');
         }
-        $release = $this->release($version);
 
-        $home = getenv('HOME') ?: (string) ($_SERVER['HOME'] ?? '');
-        $defaultPrefix = rtrim($home, '/') . '/.typephp';
         $prefix = $this->expandHome($this->console->ask("Install directory [{$defaultPrefix}]: ", $defaultPrefix), $home);
-        if ($this->hasLibPhp($prefix) && $this->installedVersion($prefix) === $version
-            && $this->console->confirm("PHP {$version} with libphp.so already exists in {$prefix}; use it?", true)) {
-            putenv('PHP_HOME=' . $prefix);
-            $_ENV['PHP_HOME'] = $prefix;
-            return $prefix;
-        }
-        $this->install($release, $prefix);
+        // The offer above covered only the default directory and the running
+        // PHP; the answers just given may name another build.
+        if (($prefix !== $defaultPrefix || $version !== $defaultVersion)
+            && $this->offerInstalled($prefix, $version)) {
+            return $this->activate($prefix);
+        }
+        // Reaching php.net is pointless until a build is known to be needed.
+        $this->install($this->release($version), $prefix);
+        return $this->activate($prefix);
+    }
+
+    private function offerInstalled(string $prefix, string $version): bool
+    {
+        return $this->hasLibPhp($prefix)
+            && $this->installedVersion($prefix) === $version
+            && $this->console->confirm("PHP {$version} with libphp.so already exists in {$prefix}; use it?", true);
+    }
+
+    /** Point this process, and the build it runs, at the selected installation. */
+    private function activate(string $prefix): string
+    {
         putenv('PHP_HOME=' . $prefix);
         $_ENV['PHP_HOME'] = $prefix;
         return $prefix;
diff --git a/src/Installer/PhpBuildConfiguration.php b/src/Installer/PhpBuildConfiguration.php
index afc8d766..e0de4504 100644
--- a/src/Installer/PhpBuildConfiguration.php
+++ b/src/Installer/PhpBuildConfiguration.php
@@ -81,6 +81,34 @@ final class PhpBuildConfiguration
         return $options;
     }
 
+    /**
+     * Autoconf installation directories, program name transforms and cache files.
+     *
+     * A distribution build points these outside its own --prefix (--mandir=/usr/share/man,
+     * --includedir=/usr/include) or renames the installed binaries (--program-suffix=8.3).
+     * Inheriting them makes `make install` write to system paths the user cannot own,
+     * and hides bin/php behind a versioned name. Autoconf derives every one of them
+     * from --prefix when it is absent, so dropping them keeps the private build
+     * entirely inside the requested prefix.
+     *
+     * The versioned name is the Debian and Ubuntu packaging scheme, not a PPA
+     * addition: php8.3-dev in noble-updates/main carries --program-suffix=8.3
+     * and --mandir=/usr/share/man, and ppa:ondrej/php repeats it per version.
+     *
+     * @var list<string>
+     */
+    private const array PREFIX_DERIVED = [
+        '--exec-prefix', '--bindir', '--sbindir', '--libexecdir', '--sysconfdir',
+        '--sharedstatedir', '--localstatedir', '--runstatedir', '--libdir',
+        '--includedir', '--oldincludedir', '--datarootdir', '--datadir',
+        '--infodir', '--localedir', '--mandir', '--docdir', '--htmldir',
+        '--dvidir', '--pdfdir', '--psdir',
+        '--program-prefix', '--program-suffix', '--program-transform-name',
+        // A cache recorded for the distribution prefix answers the wrong questions
+        // here, and its path may not even exist on this machine.
+        '--cache-file', '--config-cache',
+    ];
+
     /**
      * @param string|list<string> $configureOptions
      * @return list<string>
@@ -91,7 +119,10 @@ final class PhpBuildConfiguration
             '--prefix', '--with-config-file-path', '--with-config-file-scan-dir',
             '--enable-embed', '--enable-cli', '--disable-cli', '--with-libdir',
         ];
-        $drop = ['--with-apxs', '--with-apxs2', '--enable-fpm', '--with-fpm-systemd'];
+        $drop = [
+            '--with-apxs', '--with-apxs2', '--enable-fpm', '--with-fpm-systemd',
+            ...self::PREFIX_DERIVED,
+        ];
         $result = [];
         $options = is_string($configureOptions) ? self::parseShellWords($configureOptions) : $configureOptions;
         foreach ($options as $option) {