diff --git a/php/ext/google/protobuf/convert.c b/php/ext/google/protobuf/convert.c
index 31deb742d7024..20f19084a12fa 100644
--- a/php/ext/google/protobuf/convert.c
+++ b/php/ext/google/protobuf/convert.c
@@ -432,7 +432,7 @@ void Convert_UpbToPhp(upb_MessageValue upb_val, zval* php_val, TypeInfo type,
 #else
     {
       char buf[21];
-      int size = sprintf(buf, "%lld", upb_val.int64_val);
+      int size = snprintf(buf, sizeof(buf), "%lld", upb_val.int64_val);
       ZVAL_NEW_STR(php_val, zend_string_init(buf, size, 0));
     }
 #endif
@@ -443,7 +443,7 @@ void Convert_UpbToPhp(upb_MessageValue upb_val, zval* php_val, TypeInfo type,
 #else
     {
       char buf[21];
-      int size = sprintf(buf, "%lld", (int64_t)upb_val.uint64_val);
+      int size = snprintf(buf, sizeof(buf), "%lld", (int64_t)upb_val.uint64_val);
       ZVAL_NEW_STR(php_val, zend_string_init(buf, size, 0));
     }
 #endif