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
@@ -185,6 +185,7 @@
     'ArcanistPHPOpenTagXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistPHPOpenTagXHPASTLinterRule.php',
     'ArcanistPHPShortTagXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistPHPShortTagXHPASTLinterRule.php',
     'ArcanistParenthesesSpacingXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistParenthesesSpacingXHPASTLinterRule.php',
+    'ArcanistParseStrUseXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistParseStrUseXHPASTLinterRule.php',
     'ArcanistPasteWorkflow' => 'workflow/ArcanistPasteWorkflow.php',
     'ArcanistPatchWorkflow' => 'workflow/ArcanistPatchWorkflow.php',
     'ArcanistPhpLinter' => 'lint/linter/ArcanistPhpLinter.php',
@@ -467,6 +468,7 @@
     'ArcanistPHPOpenTagXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistPHPShortTagXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistParenthesesSpacingXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
+    'ArcanistParseStrUseXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistPasteWorkflow' => 'ArcanistWorkflow',
     'ArcanistPatchWorkflow' => 'ArcanistWorkflow',
     'ArcanistPhpLinter' => 'ArcanistExternalLinter',
diff --git a/src/lint/linter/__tests__/xhpast/extract-use.lint-test b/src/lint/linter/__tests__/xhpast/extract-use.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/__tests__/xhpast/extract-use.lint-test
@@ -0,0 +1,5 @@
+<?php
+
+extract();
+~~~~~~~~~~
+error:3:1
diff --git a/src/lint/linter/__tests__/xhpast/parse_str-use.lint-test b/src/lint/linter/__tests__/xhpast/parse_str-use.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/__tests__/xhpast/parse_str-use.lint-test
@@ -0,0 +1,7 @@
+<?php
+
+$params = array();
+parse_str('foo=bar', $params);
+parse_str('foo=bar');
+~~~~~~~~~~
+error:5:1
diff --git a/src/lint/linter/xhpast/rules/ArcanistParseStrUseXHPASTLinterRule.php b/src/lint/linter/xhpast/rules/ArcanistParseStrUseXHPASTLinterRule.php
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/ArcanistParseStrUseXHPASTLinterRule.php
@@ -0,0 +1,29 @@
+<?php
+
+final class ArcanistParseStrUseXHPASTLinterRule
+  extends ArcanistXHPASTLinterRule {
+
+  const ID = 80;
+
+  public function getLintName() {
+    return pht('Questionable Use of %s', 'parse_str()');
+  }
+
+  public function process(XHPASTNode $root) {
+    $calls = $this->getFunctionCalls($root, array('parse_str'));
+
+    foreach ($calls as $call) {
+      $call_params = $call->getChildOfType(1, 'n_CALL_PARAMETER_LIST');
+
+      if (count($call_params->getChildren()) < 2) {
+        $this->raiseLintAtNode(
+          $call,
+          pht(
+            'Avoid %s unless the second parameter is specified. '.
+            'It is confusing and hinders static analysis.',
+            'parse_str()'));
+      }
+    }
+  }
+
+}
diff --git a/src/lint/linter/xhpast/rules/ArcanistUndeclaredVariableXHPASTLinterRule.php b/src/lint/linter/xhpast/rules/ArcanistUndeclaredVariableXHPASTLinterRule.php
--- a/src/lint/linter/xhpast/rules/ArcanistUndeclaredVariableXHPASTLinterRule.php
+++ b/src/lint/linter/xhpast/rules/ArcanistUndeclaredVariableXHPASTLinterRule.php
@@ -48,6 +48,8 @@
     //
     // TODO: Support functions defined inside other functions which is commonly
     // used with anonymous functions.
+    //
+    // TODO: parse_str() also makes lexical scope unknowable, see D13857.
 
     $defs = $root->selectDescendantsOfTypes(array(
       'n_FUNCTION_DECLARATION',