<?php
function padIntegerPart($n, $len) {
    $intPart = (int)$n;

    return str_repeat('0', max(0, $len - 1 - floor(log($intPart, 10)))) . $n;
}

echo padIntegerPart(2.11, 1)."\n";
echo padIntegerPart(2.11, 2)."\n";
echo padIntegerPart(2.11, 3)."\n";
?>

2.11
02.11
002.11