def is_placeholder(x):
    """Returns whether `x` is a placeholder.

    # Arguments
        x: A candidate placeholder.

    # Returns
        Boolean.

    # Examples
    ```python
        >>> from keras import backend as K
        >>> input_ph = K.placeholder(shape=(2,4,5))
        >>> K.is_placeholder(input_ph)
        True
    ```
    """
    return hasattr(x, '_mxnet_placeholder') and x._mxnet_placeholder
