diff --git a/src/error/PhutilErrorHandler.php b/src/error/PhutilErrorHandler.php
--- a/src/error/PhutilErrorHandler.php
+++ b/src/error/PhutilErrorHandler.php
@@ -181,7 +181,6 @@
    * @task internal
    */
   public static function handleError($num, $str, $file, $line, $ctx = null) {
-
     foreach (self::$traps as $trap) {
       $trap->addError($num, $str, $file, $line);
     }
@@ -378,7 +377,7 @@
    * @task internal
    */
   public static function dispatchErrorMessage($event, $value, $metadata) {
-    $timestamp = strftime('%Y-%m-%d %H:%M:%S');
+    $timestamp = date('Y-m-d H:i:s');
 
     switch ($event) {
       case self::ERROR:
diff --git a/src/filesystem/linesofalarge/LinesOfALarge.php b/src/filesystem/linesofalarge/LinesOfALarge.php
--- a/src/filesystem/linesofalarge/LinesOfALarge.php
+++ b/src/filesystem/linesofalarge/LinesOfALarge.php
@@ -212,7 +212,7 @@
         if (strlen($this->buf)) {
           $this->num++;
           $this->line = $this->buf;
-          $this->buf = null;
+          $this->buf = '';
         } else {
           $this->valid = false;
         }
diff --git a/src/future/exec/ExecFuture.php b/src/future/exec/ExecFuture.php
--- a/src/future/exec/ExecFuture.php
+++ b/src/future/exec/ExecFuture.php
@@ -199,7 +199,7 @@
       (string)substr($this->stderr, $this->stderrPos),
     );
 
-    $this->stderrPos = strlen($this->stderr);
+    $this->stderrPos = $this->getStderrBufferLength();
 
     return $result;
   }
@@ -210,7 +210,8 @@
     }
 
     $result = (string)substr($this->stdout, $this->stdoutPos);
-    $this->stdoutPos = strlen($this->stdout);
+    $this->stdoutPos = $this->getStdoutBufferLength();
+
     return $result;
   }
 
@@ -475,7 +476,7 @@
    * @task internal
    */
   public function isReadBufferEmpty() {
-    return !strlen($this->stdout);
+    return !$this->getStdoutBufferLength();
   }
 
 
@@ -757,14 +758,17 @@
     $max_stdout_read_bytes = PHP_INT_MAX;
     $max_stderr_read_bytes = PHP_INT_MAX;
     if ($read_buffer_size !== null) {
-      $max_stdout_read_bytes = $read_buffer_size - strlen($this->stdout);
-      $max_stderr_read_bytes = $read_buffer_size - strlen($this->stderr);
+      $stdout_len = $this->getStdoutBufferLength();
+      $stderr_len = $this->getStderrBufferLength();
+
+      $max_stdout_read_bytes = $read_buffer_size - $stdout_len;
+      $max_stderr_read_bytes = $read_buffer_size - $stderr_len;
     }
 
     if ($max_stdout_read_bytes > 0) {
       $this->stdout .= $this->readAndDiscard(
         $stdout,
-        $this->getStdoutSizeLimit() - strlen($this->stdout),
+        $this->getStdoutSizeLimit() - $this->getStdoutBufferLength(),
         'stdout',
         $max_stdout_read_bytes);
     }
@@ -772,7 +776,7 @@
     if ($max_stderr_read_bytes > 0) {
       $this->stderr .= $this->readAndDiscard(
         $stderr,
-        $this->getStderrSizeLimit() - strlen($this->stderr),
+        $this->getStderrSizeLimit() - $this->getStderrBufferLength(),
         'stderr',
         $max_stderr_read_bytes);
     }
@@ -1013,5 +1017,20 @@
     );
   }
 
+  private function getStdoutBufferLength() {
+    if ($this->stdout === null) {
+      return 0;
+    }
+
+    return strlen($this->stdout);
+  }
+
+  private function getStderrBufferLength() {
+    if ($this->stderr === null) {
+      return 0;
+    }
+
+    return strlen($this->stderr);
+  }
 
 }
diff --git a/src/future/http/BaseHTTPFuture.php b/src/future/http/BaseHTTPFuture.php
--- a/src/future/http/BaseHTTPFuture.php
+++ b/src/future/http/BaseHTTPFuture.php
@@ -206,12 +206,14 @@
    * @task config
    */
   public function getHeaders($filter = null) {
-    $filter = strtolower($filter);
+    if ($filter !== null) {
+      $filter = phutil_utf8_strtolower($filter);
+    }
 
     $result = array();
     foreach ($this->headers as $header) {
       list($name, $value) = $header;
-      if (!$filter || ($filter == strtolower($name))) {
+      if (($filter === null) || ($filter === phutil_utf8_strtolower($name))) {
         $result[] = $header;
       }
     }
diff --git a/src/future/http/HTTPSFuture.php b/src/future/http/HTTPSFuture.php
--- a/src/future/http/HTTPSFuture.php
+++ b/src/future/http/HTTPSFuture.php
@@ -269,7 +269,7 @@
         curl_setopt($curl, CURLOPT_REDIR_PROTOCOLS, $allowed_protocols);
       }
 
-      if (strlen($this->rawBody)) {
+      if ($this->rawBody !== null) {
         if ($this->getData()) {
           throw new Exception(
             pht(
diff --git a/src/lint/linter/xhpast/rules/ArcanistCommentStyleXHPASTLinterRule.php b/src/lint/linter/xhpast/rules/ArcanistCommentStyleXHPASTLinterRule.php
--- a/src/lint/linter/xhpast/rules/ArcanistCommentStyleXHPASTLinterRule.php
+++ b/src/lint/linter/xhpast/rules/ArcanistCommentStyleXHPASTLinterRule.php
@@ -17,6 +17,14 @@
         continue;
       }
 
+      // Don't warn about PHP comment directives. In particular, we need
+      // to use "#[\ReturnTypeWillChange]" to implement "Iterator" in a way
+      // that is compatible with PHP 8.1 and older versions of PHP prior
+      // to the introduction of return types. See T13588.
+      if (preg_match('/^#\\[\\\\/', $value)) {
+        continue;
+      }
+
       $this->raiseLintAtOffset(
         $comment->getOffset(),
         pht(
diff --git a/src/lint/linter/xhpast/rules/__tests__/comment-style/hash-directives.lint-test b/src/lint/linter/xhpast/rules/__tests__/comment-style/hash-directives.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/__tests__/comment-style/hash-directives.lint-test
@@ -0,0 +1,23 @@
+<?php
+
+class X implements Iterator {
+
+  #[\ReturnTypeWillChange]
+  public function reset() {
+    # See T13588 for PHP8.1 compatibility information.
+  }
+
+}
+~~~~~~~~~~
+error:7:5:XHP18:Comment Style
+~~~~~~~~~~
+<?php
+
+class X implements Iterator {
+
+  #[\ReturnTypeWillChange]
+  public function reset() {
+    // See T13588 for PHP8.1 compatibility information.
+  }
+
+}
diff --git a/src/object/Phobject.php b/src/object/Phobject.php
--- a/src/object/Phobject.php
+++ b/src/object/Phobject.php
@@ -33,22 +33,27 @@
         get_class($this).'::'.$name));
   }
 
+  #[\ReturnTypeWillChange]
   public function current() {
     $this->throwOnAttemptedIteration();
   }
 
+  #[\ReturnTypeWillChange]
   public function key() {
     $this->throwOnAttemptedIteration();
   }
 
+  #[\ReturnTypeWillChange]
   public function next() {
     $this->throwOnAttemptedIteration();
   }
 
+  #[\ReturnTypeWillChange]
   public function rewind() {
     $this->throwOnAttemptedIteration();
   }
 
+  #[\ReturnTypeWillChange]
   public function valid() {
     $this->throwOnAttemptedIteration();
   }
diff --git a/src/parser/PhutilURI.php b/src/parser/PhutilURI.php
--- a/src/parser/PhutilURI.php
+++ b/src/parser/PhutilURI.php
@@ -166,7 +166,7 @@
     if ($this->isGitURI()) {
       $protocol = null;
     } else {
-      if (strlen($auth)) {
+      if ($auth !== null) {
         $protocol = nonempty($this->protocol, 'http');
       }
     }
diff --git a/src/parser/argument/PhutilArgumentParser.php b/src/parser/argument/PhutilArgumentParser.php
--- a/src/parser/argument/PhutilArgumentParser.php
+++ b/src/parser/argument/PhutilArgumentParser.php
@@ -769,7 +769,12 @@
         pht('There is no **%s** workflow.', $workflow_name));
     } else {
       $out[] = $this->indent($indent, $workflow->getExamples());
-      $out[] = $this->indent($indent, $workflow->getSynopsis());
+
+      $synopsis = $workflow->getSynopsis();
+      if ($synopsis !== null) {
+        $out[] = $this->indent($indent, $workflow->getSynopsis());
+      }
+
       if ($show_details) {
         $full_help = $workflow->getHelp();
         if ($full_help) {
diff --git a/src/symbols/PhutilClassMapQuery.php b/src/symbols/PhutilClassMapQuery.php
--- a/src/symbols/PhutilClassMapQuery.php
+++ b/src/symbols/PhutilClassMapQuery.php
@@ -226,8 +226,8 @@
     $unique = $this->uniqueMethod;
     $sort = $this->sortMethod;
 
-    if (strlen($expand)) {
-      if (!strlen($unique)) {
+    if ($expand !== null) {
+      if ($unique === null) {
         throw new Exception(
           pht(
             'Trying to execute a class map query for descendants of class '.
@@ -245,7 +245,7 @@
       ->loadObjects();
 
     // Apply the "expand" mechanism, if it is configured.
-    if (strlen($expand)) {
+    if ($expand !== null) {
       $list = array();
       foreach ($objects as $object) {
         foreach (call_user_func(array($object, $expand)) as $instance) {
@@ -257,7 +257,7 @@
     }
 
     // Apply the "unique" mechanism, if it is configured.
-    if (strlen($unique)) {
+    if ($unique !== null) {
       $map = array();
       foreach ($list as $object) {
         $key = call_user_func(array($object, $unique));
@@ -287,12 +287,12 @@
     }
 
     // Apply the "filter" mechanism, if it is configured.
-    if (strlen($filter)) {
+    if ($filter !== null) {
       $map = mfilter($map, $filter);
     }
 
     // Apply the "sort" mechanism, if it is configured.
-    if (strlen($sort)) {
+    if ($sort !== null) {
       if ($map) {
         // The "sort" method may return scalars (which we want to sort with
         // "msort()"), or may return PhutilSortVector objects (which we want
diff --git a/src/workflow/ArcanistWorkflow.php b/src/workflow/ArcanistWorkflow.php
--- a/src/workflow/ArcanistWorkflow.php
+++ b/src/workflow/ArcanistWorkflow.php
@@ -143,7 +143,7 @@
 
     if ($information) {
       $synopsis = $information->getSynopsis();
-      if (strlen($synopsis)) {
+      if ($synopsis !== null) {
         $phutil_workflow->setSynopsis($synopsis);
       }