diff --git a/src/lint/linter/ArcanistXHPASTLinter.php b/src/lint/linter/ArcanistXHPASTLinter.php
--- a/src/lint/linter/ArcanistXHPASTLinter.php
+++ b/src/lint/linter/ArcanistXHPASTLinter.php
@@ -58,6 +58,7 @@
   const LINT_UNNECESSARY_SEMICOLON      = 56;
   const LINT_SELF_MEMBER_REFERENCE      = 57;
   const LINT_LOGICAL_OPERATORS          = 58;
+  const LINT_LOWERCASE_FUNCTIONS        = 59;
 
   private $blacklistedFunctions = array();
   private $naminghook;
@@ -129,6 +130,7 @@
       self::LINT_UNNECESSARY_SEMICOLON      => 'Unnecessary Semicolon',
       self::LINT_SELF_MEMBER_REFERENCE      => 'Self Member Reference',
       self::LINT_LOGICAL_OPERATORS          => 'Logical Operators',
+      self::LINT_LOWERCASE_FUNCTIONS        => 'Lowercase Functions',
     );
   }
 
@@ -175,6 +177,7 @@
       self::LINT_UNNECESSARY_SEMICOLON      => $advice,
       self::LINT_SELF_MEMBER_REFERENCE      => $advice,
       self::LINT_LOGICAL_OPERATORS          => $advice,
+      self::LINT_LOWERCASE_FUNCTIONS        => $advice,
     );
   }
 
@@ -242,7 +245,7 @@
 
   public function getVersion() {
     // The version number should be incremented whenever a new rule is added.
-    return '20';
+    return '21';
   }
 
   protected function resolveFuture($path, Future $future) {
@@ -324,6 +327,7 @@
       'lintUnnecessarySemicolons' => self::LINT_UNNECESSARY_SEMICOLON,
       'lintSelfMemberReference' => self::LINT_SELF_MEMBER_REFERENCE,
       'lintLogicalOperators' => self::LINT_LOGICAL_OPERATORS,
+      'lintLowercaseFunctions' => self::LINT_LOWERCASE_FUNCTIONS,
     );
 
     foreach ($method_codes as $method => $codes) {
@@ -3430,6 +3434,39 @@
     }
   }
 
+  private function lintLowercaseFunctions(XHPASTNode $root) {
+    static $builtin_functions = null;
+
+    if ($builtin_functions === null) {
+      $builtin_functions = array_fuse(
+        idx(get_defined_functions(), 'internal', array()));
+    }
+
+    $function_calls = $root->selectDescendantsOfType('n_FUNCTION_CALL');
+
+    foreach ($function_calls as $function_call) {
+      $function = $function_call->getChildByIndex(0);
+
+      if ($function->getTypeName() != 'n_SYMBOL_NAME') {
+        continue;
+      }
+
+      $function_name = $function->getConcreteString();
+
+      if (!idx($builtin_functions, strtolower($function_name))) {
+        continue;
+      }
+
+      if ($function_name != strtolower($function_name)) {
+        $this->raiseLintAtNode(
+          $function,
+          self::LINT_LOWERCASE_FUNCTIONS,
+          pht('Calls to built-in PHP functions should be lowercase.'),
+          strtolower($function_name));
+      }
+    }
+  }
+
   public function getSuperGlobalNames() {
     return array(
       '$GLOBALS',
diff --git a/src/lint/linter/__tests__/xhpast/lowercase-functions.lint-test b/src/lint/linter/__tests__/xhpast/lowercase-functions.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/__tests__/xhpast/lowercase-functions.lint-test
@@ -0,0 +1,9 @@
+<?php
+var_dump('');
+Var_Dump('');
+~~~~~~~~~~
+advice:3:1
+~~~~~~~~~~
+<?php
+var_dump('');
+var_dump('');