diff --git a/thirdparty/php-peg/cli.php b/thirdparty/php-peg/cli.php
index 5a2b3c703f2..1bfb0322a33 100644
--- a/thirdparty/php-peg/cli.php
+++ b/thirdparty/php-peg/cli.php
@@ -5,4 +5,10 @@
 
 use hafriedlander\Peg\Compiler;
 
+// Just in case this matters.
+/// https://github.com/hafriedlander/php-peg/issues/39
+if (php_sapi_name() != "cli") {
+  die("Maybe bad...");
+}
+
 Compiler::cli( $_SERVER['argv'] ) ;