$_POST to EE template not working as expected

Question

Using this form to post to itself is not working as expected.

But why?

    <form method='POST' action='gen_req'>
      <input id='input' name='uid' placeholder='uid' />
      <button class='btn'>Go</button>
    </form>

    {exp:member:custom_profile_data member_id="<? echo $_POST['uid']; ?>"}
      <p>{email}</p>
    {/exp:member:custom_profile_data}

UPDATED

I have corrected to add the XID line and use the ee() for the post. But still not returning any data.

This is what i have updated to:

<form method='POST' action='test2'>
  <input type="hidden" name="XID" value="{XID_HASH}" />  
  <input id='input' name='uid' placeholder='uid' />
  <button class='btn'>Go</button>
</form>

{exp:member:custom_profile_data member_id="<? echo ee()->input->post('uid', TRUE); ?>"}
  <p>{email}</p>
{/exp:member:custom_profile_data}

Answer 1

Do you have PHP Parsing Stage on "Input"?

<form method='POST' action='gen_req'>
  <input id='input' name='uid' placeholder='uid' />
  <button class='btn'>Go</button>
</form>

{exp:member:custom_profile_data member_id="<? echo ee()->input->post('uid', TRUE); ?>"}
  <p>{email}</p>
{/exp:member:custom_profile_data}