<?php

// Arbitrary default percentage and method.
$default_percentage = 100;

// Get POST value.
$percentage = $_POST['percentage'] ? $_POST['percentage'] : $default_percentage;

// Function to test percentage.
function is_limited_percentage( $perc, $lower, $upper )
{
  return $perc >= $lower && $perc <= $upper;
}

if( is_limited_percentage( $percentage, 0, 100 ))
{
  echo 'Percentage is between 0 and 100';
} else
{
  echo 'Percentage is invalid';
}

/* End of script */