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
@@ -528,6 +528,7 @@
     'nonempty' => 'utils/utils.php',
     'phlog' => 'error/phlog.php',
     'pht' => 'internationalization/pht.php',
+    'phutil_build_http_querystring' => 'utils/utils.php',
     'phutil_censor_credentials' => 'utils/utils.php',
     'phutil_console_confirm' => 'console/format.php',
     'phutil_console_format' => 'console/format.php',
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
@@ -273,7 +273,7 @@
       return strlen($data);
     }
 
-    return strlen(http_build_query($data, '', '&'));
+    return strlen(phutil_build_http_querystring($data));
   }
 
 
diff --git a/src/future/http/HTTPFuture.php b/src/future/http/HTTPFuture.php
--- a/src/future/http/HTTPFuture.php
+++ b/src/future/http/HTTPFuture.php
@@ -244,7 +244,7 @@
 
     if ($this->getMethod() == 'GET') {
       if (is_array($data)) {
-        $data = http_build_query($data, '', '&');
+        $data = phutil_build_http_querystring($data);
         if (strpos($uri, '?') !== false) {
           $uri .= '&'.$data;
         } else {
@@ -254,7 +254,7 @@
       }
     } else {
       if (is_array($data)) {
-        $data = http_build_query($data, '', '&')."\r\n";
+        $data = phutil_build_http_querystring($data)."\r\n";
         $add_headers[] = array(
           'Content-Type',
           'application/x-www-form-urlencoded',
diff --git a/src/future/http/HTTPSFuture.php b/src/future/http/HTTPSFuture.php
--- a/src/future/http/HTTPSFuture.php
+++ b/src/future/http/HTTPSFuture.php
@@ -523,7 +523,7 @@
       // If we don't have any files, just encode the data as a query string,
       // make sure it's not including any files, and we're good to go.
       if (is_array($data)) {
-        $data = http_build_query($data, '', '&');
+        $data = phutil_build_http_querystring($data);
       }
 
       $this->checkForDangerousCURLMagic($data, $is_query_string = true);
diff --git a/src/markup/engine/remarkup/blockrule/PhutilRemarkupTestInterpreterRule.php b/src/markup/engine/remarkup/blockrule/PhutilRemarkupTestInterpreterRule.php
--- a/src/markup/engine/remarkup/blockrule/PhutilRemarkupTestInterpreterRule.php
+++ b/src/markup/engine/remarkup/blockrule/PhutilRemarkupTestInterpreterRule.php
@@ -11,7 +11,7 @@
     return sprintf(
       "Content: (%s)\nArgv: (%s)",
       $content,
-      http_build_query($argv));
+      phutil_build_http_querystring($argv));
   }
 
 }
diff --git a/src/parser/PhutilURI.php b/src/parser/PhutilURI.php
--- a/src/parser/PhutilURI.php
+++ b/src/parser/PhutilURI.php
@@ -174,7 +174,7 @@
     }
 
     if ($this->query) {
-      $query = '?'.http_build_query($this->query, '', '&');
+      $query = '?'.phutil_build_http_querystring($this->query);
     } else {
       $query = null;
     }
diff --git a/src/utils/__tests__/PhutilUtilsTestCase.php b/src/utils/__tests__/PhutilUtilsTestCase.php
--- a/src/utils/__tests__/PhutilUtilsTestCase.php
+++ b/src/utils/__tests__/PhutilUtilsTestCase.php
@@ -882,4 +882,43 @@
     return array_select_keys($map, $keys);
   }
 
+  public function testQueryStringEncoding() {
+    $expect = array();
+
+    // As a starting point, we expect every character to encode as an "%XX"
+    // escaped version.
+    foreach (range(0, 255) as $byte) {
+      $c = chr($byte);
+      $expect[$c] = sprintf('%%%02X', $byte);
+    }
+
+    // We expect these characters to not be escaped.
+    $ranges = array(
+      range('a', 'z'),
+      range('A', 'Z'),
+      range('0', '9'),
+      array('-', '.', '_', '~'),
+    );
+
+    foreach ($ranges as $range) {
+      foreach ($range as $preserve_char) {
+        $expect[$preserve_char] = $preserve_char;
+      }
+    }
+
+    foreach (range(0, 255) as $byte) {
+      $c = chr($byte);
+
+      $expect_c = $expect[$c];
+      $expect_str = "{$expect_c}={$expect_c}";
+
+      $actual_str = phutil_build_http_querystring(array($c => $c));
+
+      $this->assertEqual(
+        $expect_str,
+        $actual_str,
+        pht('HTTP querystring for byte "%s".', sprintf('0x%02x', $byte)));
+    }
+  }
+
 }
diff --git a/src/utils/utils.php b/src/utils/utils.php
--- a/src/utils/utils.php
+++ b/src/utils/utils.php
@@ -1537,3 +1537,24 @@
 
   return ($bits === 0);
 }
+
+
+/**
+ * Build a query string from a dictionary.
+ *
+ * @param map<string, string> Dictionary of parameters.
+ * @return string HTTP query string.
+ */
+function phutil_build_http_querystring(array $parameters) {
+  // We want to encode in RFC3986 mode, but "http_build_query()" did not get
+  // a flag for that mode until PHP 5.4.0. This is equivalent to calling
+  // "http_build_query()" with the "PHP_QUERY_RFC3986" flag.
+
+  $query = array();
+  foreach ($parameters as $key => $value) {
+    $query[] = rawurlencode($key).'='.rawurlencode($value);
+  }
+  $query = implode($query, '&');
+
+  return $query;
+}