--- a/src/google/protobuf/compiler/php/php_generator.cc
+++ b/src/google/protobuf/compiler/php/php_generator.cc
@@ -143,7 +143,7 @@ TProtoStringType ConstantNamePrefix(absl::string_view classname) {
   bool is_reserved = false;
 
   TProtoStringType lower(classname);
-  std::transform(lower.begin(), lower.end(), lower.begin(), ::tolower);
+  lower.to_lower();
 
   is_reserved = IsReservedName(lower);
 
@@ -1414,7 +1414,7 @@ void GenerateEnumFile(const FileDescriptor* file, const EnumDescriptor* en,
 
   // Write legacy file for backwards compatibility with "readonly" keywword
   TProtoStringType lower = en->name();
-  std::transform(lower.begin(), lower.end(), lower.begin(), ::tolower);
+  lower.to_lower();
   if (lower == "readonly") {
     printer.Print(
         "// Adding a class alias for backwards compatibility with the \"readonly\" keyword.\n");
@@ -1541,7 +1541,7 @@ void GenerateMessageFile(const FileDescriptor* file, const Descriptor* message,
 
   // Write legacy file for backwards compatibility with "readonly" keywword
   TProtoStringType lower = message->name();
-  std::transform(lower.begin(), lower.end(), lower.begin(), ::tolower);
+  lower.to_lower();
   if (lower == "readonly") {
     printer.Print(
         "// Adding a class alias for backwards compatibility with the \"readonly\" keyword.\n");

--- a/src/google/protobuf/compiler/php/names.cc
+++ b/src/google/protobuf/compiler/php/names.cc
@@ -63,7 +63,7 @@ namespace php {
 
 bool IsReservedName(absl::string_view name) {
   TProtoStringType lower(name);
-  std::transform(lower.begin(), lower.end(), lower.begin(), ::tolower);
+  lower.to_lower();
   for (int i = 0; i < kReservedNamesSize; i++) {
     if (lower == kReservedNames[i]) {
       return true;