diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -744,6 +744,7 @@
     'PhutilEnglishCanadaLocale' => 'internationalization/locales/PhutilEnglishCanadaLocale.php',
     'PhutilErrorHandler' => 'error/PhutilErrorHandler.php',
     'PhutilErrorHandlerTestCase' => 'error/__tests__/PhutilErrorHandlerTestCase.php',
+    'PhutilErrorLog' => 'filesystem/PhutilErrorLog.php',
     'PhutilErrorTrap' => 'error/PhutilErrorTrap.php',
     'PhutilEvent' => 'events/PhutilEvent.php',
     'PhutilEventConstants' => 'events/constant/PhutilEventConstants.php',
@@ -1816,6 +1817,7 @@
     'PhutilEnglishCanadaLocale' => 'PhutilLocale',
     'PhutilErrorHandler' => 'Phobject',
     'PhutilErrorHandlerTestCase' => 'PhutilTestCase',
+    'PhutilErrorLog' => 'Phobject',
     'PhutilErrorTrap' => 'Phobject',
     'PhutilEvent' => 'Phobject',
     'PhutilEventConstants' => 'Phobject',
diff --git a/src/filesystem/Filesystem.php b/src/filesystem/Filesystem.php
--- a/src/filesystem/Filesystem.php
+++ b/src/filesystem/Filesystem.php
@@ -52,7 +52,7 @@
    * Make assertions about the state of path in preparation for
    * writeFile() and writeFileIfChanged().
    */
-  private static function assertWritableFile($path) {
+  public static function assertWritableFile($path) {
     $path = self::resolvePath($path);
     $dir = dirname($path);
 
diff --git a/src/filesystem/PhutilErrorLog.php b/src/filesystem/PhutilErrorLog.php
new file mode 100644
--- /dev/null
+++ b/src/filesystem/PhutilErrorLog.php
@@ -0,0 +1,101 @@
+<?php
+
+
+final class PhutilErrorLog
+  extends Phobject {
+
+  private $logName;
+  private $logPath;
+
+  public function setLogName($log_name) {
+    $this->logName = $log_name;
+    return $this;
+  }
+
+  public function getLogName() {
+    return $this->logName;
+  }
+
+  public function setLogPath($log_path) {
+    $this->logPath = $log_path;
+    return $this;
+  }
+
+  public function getLogPath() {
+    return $this->logPath;
+  }
+
+  public function activateLog() {
+    $log_path = $this->getLogPath();
+
+    if ($log_path !== null) {
+      // Test that the path is writable.
+      $write_exception = null;
+      try {
+        Filesystem::assertWritableFile($log_path);
+      } catch (FilesystemException $ex) {
+        $write_exception = $ex;
+      }
+
+      // If we hit an exception, try to create the containing directory.
+      if ($write_exception) {
+        $log_dir = dirname($log_path);
+        if (!Filesystem::pathExists($log_dir)) {
+          try {
+            Filesystem::createDirectory($log_dir, 0755, true);
+          } catch (FilesystemException $ex) {
+            throw new PhutilProxyException(
+              pht(
+                'Unable to write log "%s" to path "%s". The containing '.
+                'directory ("%s") does not exist or is not readable, and '.
+                'could not be created.',
+                $this->getLogName(),
+                $log_path,
+                $log_dir),
+              $ex);
+          }
+        }
+
+        // If we created the parent directory, test if the path is writable
+        // again.
+        try {
+          Filesystem::assertWritableFile($log_path);
+          $write_exception = null;
+        } catch (FilesystemException $ex) {
+          $write_exception = $ex;
+        }
+      }
+
+      // If we ran into a write exception and couldn't resolve it, fail.
+      if ($write_exception) {
+        throw new PhutilProxyException(
+          pht(
+            'Unable to write log "%s" to path "%s" because the path is not '.
+            'writable.',
+            $this->getLogName(),
+            $log_path),
+          $write_exception);
+      }
+    }
+
+    ini_set('error_log', $log_path);
+    PhutilErrorHandler::setErrorListener(array($this, 'onError'));
+  }
+
+  public function onError($event, $value, array $metadata) {
+    // If we've set "error_log" to a real file, so messages won't be output to
+    // stderr by default. Copy them to stderr.
+
+    if ($this->logPath === null) {
+      return;
+    }
+
+    $message = idx($metadata, 'default_message');
+
+    if (strlen($message)) {
+      $message = tsprintf("%B\n", $message);
+      @fwrite(STDERR, $message);
+    }
+  }
+
+}