Online PHP editor | output for fenL6

<?php unset($_GET["action"]); echo "value: "; var_dump($_GET["action"] ?? NULL); echo "test: "; var_dump(($_GET["action"] ?? NULL) === "submit"); echo "branch: "; if (($_GET["action"] ?? NULL) === "submit") { echo "submit"; } else { echo "not submit"; } echo PHP_EOL; echo PHP_EOL; $_GET["action"] = "submit"; echo "value: "; var_dump($_GET["action"] ?? NULL); echo "test: "; var_dump(($_GET["action"] ?? NULL) === "submit"); echo "branch: "; if (($_GET["action"] ?? NULL) === "submit") { echo "submit"; } else { echo "not submit"; } echo PHP_EOL; echo PHP_EOL; $_GET["action"] = "preview"; echo "value: "; var_dump($_GET["action"] ?? NULL); echo "test: "; var_dump(($_GET["action"] ?? NULL) === "submit"); echo "branch: "; if (($_GET["action"] ?? NULL) === "submit") { echo "submit"; } else { echo "not submit"; } echo PHP_EOL;