Better than if(count($_POST) > 0)

[eluser]veledrom[/eluser]
So which one do you prefer guys? Best practise, most reliable and fastest.

Code:

<input type="submit" name="submit_button" value="Login"  />

Code:

if($this->input->post('submit_button') == 'Login') {
// do something
}

if(! empty($_POST)) {
// do something
}

if(count($_POST) > 0)
{
// do something
}

if(! empty($_POST['submit_button']))
{
// do something
}

if(isset($_POST["submit_button"]))
{
// do something
}