system/HTTP/IncomingRequest.php
public function getOldInput(string $key)
	{
		// If the session hasn't been started, or no
		// data was previously saved, we're done.
		if (empty($_SESSION['_ci_old_input']))
		{
			return;
		}

		// Check for the value in the POST array first.
		if (isset($_SESSION['_ci_old_input']['post'][$key]))
		{
			return $_SESSION['_ci_old_input']['post'][$key];
		}

		// Next check in the GET array.
		if (isset($_SESSION['_ci_old_input']['get'][$key]))
		{
			return $_SESSION['_ci_old_input']['get'][$key];
		}

		helper('array');

		// Check for an array value in POST.
		if (isset($_SESSION['_ci_old_input']['post']))
		{
			$value = dot_array_search($key, $_SESSION['_ci_old_input']['post']);
			if (! is_null($value))
			{
				return $value;
			}
		}

		// Check for an array value in GET.
		if (isset($_SESSION['_ci_old_input']['get']))
		{
			$value = dot_array_search($key, $_SESSION['_ci_old_input']['get']);
			if (! is_null($value))
			{
				return $value;
			}
		}

		//      // return null if requested session key not found
		//      return null;
	}
public function getOldInput(string $key)
	{
		// If the session hasn't been started, or no
		// data was previously saved, we're done.
		if (empty($_SESSION['_ci_old_input']))
		{
			return;
		}

		// Check for the value in the POST array first.
		if (isset($_SESSION['_ci_old_input']['post'][$key]))
		{
			return $_SESSION['_ci_old_input']['post'][$key];
		}

		// Next check in the GET array.
		if (isset($_SESSION['_ci_old_input']['get'][$key]))
		{
			return $_SESSION['_ci_old_input']['get'][$key];
		}

		helper('array');

		// Check for an array value in POST.
		if (isset($_SESSION['_ci_old_input']['post']))
		{
			$value = dot_array_search($key, $_SESSION['_ci_old_input']['post']);
			if (! is_null($value))
			{
				return $value;
			}
		}

		// Check for an array value in GET.
		if (isset($_SESSION['_ci_old_input']['get']))
		{
			$value = dot_array_search($key, $_SESSION['_ci_old_input']['get']);
			if (! is_null($value))
			{
				return $value;
			}
		}

		//      // return null if requested session key not found
		//      return null;
	}
Attempts to get old Input data that has been flashed to the session
with redirect_with_input(). It first checks for the data in the old
POST data, then the old GET data and finally check for dot arrays

@param string $key

@return mixed