core-bundle/src/Resources/contao/library/Contao/Input.php
public static function findPost($strKey)
	{
		if (isset($_POST[$strKey]))
		{
			return $_POST[$strKey];
		}

		$request = System::getContainer()->get('request_stack')->getMasterRequest();

		// Return if the session has not been started before
		if ($request === null || !$request->hasPreviousSession())
		{
			return null;
		}

		if (isset($_SESSION['FORM_DATA'][$strKey]))
		{
			return ($strKey == 'FORM_SUBMIT') ? preg_replace('/^auto_/i', '', $_SESSION['FORM_DATA'][$strKey]) : $_SESSION['FORM_DATA'][$strKey];
		}

		return null;
	}
public static function findPost($strKey)
	{
		if (isset($_POST[$strKey]))
		{
			return $_POST[$strKey];
		}

		$request = System::getContainer()->get('request_stack')->getMasterRequest();

		// Return if the session has not been started before
		if ($request === null || !$request->hasPreviousSession())
		{
			return null;
		}

		if (isset($_SESSION['FORM_DATA'][$strKey]))
		{
			return ($strKey == 'FORM_SUBMIT') ? preg_replace('/^auto_/i', '', $_SESSION['FORM_DATA'][$strKey]) : $_SESSION['FORM_DATA'][$strKey];
		}

		return null;
	}
Fallback to the session form data if there is no post data

@param string $strKey The variable name

@return mixed The variable value