diff --git a/hphp/runtime/ext/ext_string.cpp b/hphp/runtime/ext/ext_string.cpp
index 093fff12678cb2..874ac405ff99c7 100644
--- a/hphp/runtime/ext/ext_string.cpp
+++ b/hphp/runtime/ext/ext_string.cpp
@@ -276,18 +276,49 @@ Variant f_hex2bin(const String& str) {
   return ret.detach();
 }
 
-extern const StaticString s_nl;
-
 const StaticString
-  s_br("<br />\n"),
-  s_non_xhtml_br("<br>\n");
+  s_br("<br />"),
+  s_non_xhtml_br("<br>");
 
 String f_nl2br(const String& str, bool is_xhtml /* = true */) {
+  if (str.empty()) {
+    return str;
+  }
+
+  String htmlType;
   if (is_xhtml) {
-    return string_replace(str, s_nl, s_br);
+    htmlType = s_br;
   } else {
-    return string_replace(str, s_nl, s_non_xhtml_br);
+    htmlType = s_non_xhtml_br;
   }
+
+  return stringForEachBuffered(str.size(), str,
+    [&] (StringBuffer& ret, const char*& src, const char* end) {
+      // PHP treats a carriage return beside a newline as the same break
+      // no matter what order they're in.  Don't do it for two of the same in
+      // a row, though...
+      switch (*src) {
+      case '\n':
+        ret.append(htmlType);
+        // skip next if carriage return
+        if (*(src + 1) == '\r') {
+          ret.append(*src);
+          ++src;
+        }
+        ret.append(*src);
+        break;
+      case '\r':
+        ret.append(htmlType);
+        // skip next if newline
+        if (*(src + 1) == '\n') {
+          ret.append(*src);
+          ++src;
+        }
+        /* fall through */
+      default:
+        ret.append(*src);
+      }
+    });
 }
 
 String f_quotemeta(const String& str) {
diff --git a/hphp/test/quick/nl2br.php b/hphp/test/quick/nl2br.php
new file mode 100644
index 00000000000000..50c730fe26b019
--- /dev/null
+++ b/hphp/test/quick/nl2br.php
@@ -0,0 +1,22 @@
+<?php
+//Tests issue #3815
+
+//This makes reading the output easier for debugging
+function escapeNewLine($string) {
+  return str_replace(array("\n", "\r"), array('\n', '\r'), $string);
+}
+
+$stringList = array(
+  "Test\nString",
+  "Test\rString",
+  "Test\n\rString",
+  "Test\r\nString",
+  "Test\n\nString",
+  "Test\r\rString",
+  "Test String\n",
+  "Hello<br />\nmy<br />\r\nfriend<br />\n\r" //case from issue
+);
+foreach ($stringList as $string) {
+  var_dump(escapeNewLine(nl2br($string, true)));
+  var_dump(escapeNewLine(nl2br($string, false)));
+}
diff --git a/hphp/test/quick/nl2br.php.expect b/hphp/test/quick/nl2br.php.expect
new file mode 100644
index 00000000000000..ef07df642d4d84
--- /dev/null
+++ b/hphp/test/quick/nl2br.php.expect
@@ -0,0 +1,16 @@
+string(18) "Test<br />\nString"
+string(16) "Test<br>\nString"
+string(18) "Test<br />\rString"
+string(16) "Test<br>\rString"
+string(20) "Test<br />\n\rString"
+string(18) "Test<br>\n\rString"
+string(20) "Test<br />\r\nString"
+string(18) "Test<br>\r\nString"
+string(26) "Test<br />\n<br />\nString"
+string(22) "Test<br>\n<br>\nString"
+string(26) "Test<br />\r<br />\rString"
+string(22) "Test<br>\r<br>\rString"
+string(19) "Test String<br />\n"
+string(17) "Test String<br>\n"
+string(59) "Hello<br /><br />\nmy<br /><br />\r\nfriend<br /><br />\n\r"
+string(53) "Hello<br /><br>\nmy<br /><br>\r\nfriend<br /><br>\n\r"