def placeholder(shape=None, ndim=None, dtype=None, sparse=False, name=None):
"""Instantiates a placeholder tensor and returns it.
# Arguments
shape: Shape of the placeholder
(integer tuple, may include `None` entries).
ndim: Number of axes of the tensor.
At least one of {`shape`, `ndim`} must be specified.
If both are specified, `shape` is used.
dtype: Placeholder type.
name: Optional name string for the placeholder.
# Returns
Tensor instance (with Keras metadata included).
# Examples
```python
>>> from keras import backend as K
>>> input_ph = K.placeholder(shape=(2, 4, 5))
>>> input_ph._keras_shape
(2, 4, 5)
>>> input_ph
<tf.Tensor 'Placeholder_4:0' shape=(2, 4, 5) dtype=float32>
"""
if dtype is None:
dtype = floatx()
if not shape:
if ndim:
shape = tuple([None for _ in range(ndim)])
if sparse:
x = tf.sparse_placeholder(dtype, name=name)
x._dims = len(shape)
else:
x = tf.placeholder(dtype, shape=shape, name=name)
x._keras_shape = shape
x._uses_learning_phase = False
return x```
