diff --git a/src/php/tests/generated_code/math.proto b/src/php/tests/generated_code/math.proto
index 1de7d0b8debe52aa0b09a7d541fa58f6a505ea3c..c872ee6e0b2a2f499f7994d206f4eb7f10a10267 100644
--- a/src/php/tests/generated_code/math.proto
+++ b/src/php/tests/generated_code/math.proto
@@ -28,6 +28,9 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+// TODO: start using src/proto/math/math.proto and remove this file once
+// PHP supports proto3.
+
 syntax = "proto2";
 
 package math;