If you’re not specifying a page, then you would access the $_POST data on the same page as your form.
print_r( $_POST );
if ( isset( $_POST[ 'firstname' ] ) )
{
echo $_POST[ 'firstname' ] . ' ' . $_POST[ 'lastname' ];
wp_die();
}
else
{
?>
<form method="POST">
First name:<br>
<input type="text" name="firstname" value="Mickey"><br>
Last name:<br>
<input type="text" name="lastname" value="Mouse"><br><br>
<input type="submit" value="Submit">
</form>
<?php
wp_die();
}
If you want to access $_POST data on another page, you would probably need to specify that in the action attribute.
<form method="POST" action="your_page_name_here.php">
<input type="hidden" name="segment" value="test">
<input type="submit" value="Submit">
</form>