foreach( $_POST as $key => $value ){
    if( is_int($key) ) //do something with $value
}

if(count($_POST) > 1){ // just a check here.
  reset($_POST); // reset the pointer regardless of its position
  $second_value = next($_POST); // get the next value, aka 2nd element.
}

foreach ($_POST as $k => $v) {
  if (substr($k, 0, 3) == 'id_') {
    // do stuff
  }
}

<?php

$a = array(
        "first key" => "first value",
        "second key" => "second value",
);

$v = array_values($a);
echo "First value: {$v[0]}n";

?>

$ php -f a.php
First value: first value

function nth($ary, $n) {
     $b = array_slice($ary, intval($n), 1);
     return count($b) ? reset($b) : null;
}

$foo = nth($_POST, 1);

<?php
print_r($_POST);
?>

$vars = $_POST;

unset( $vars[ "known variable 1" ] );
unset( $vars[ "known variable 2" ] );

foreach($_POST as $key=>$value):
   print 'key'.$key.' value'.$value
endforeach;