Temperature conversion table generator

I'm currently learning PHP, too. This is just my latest mini-exercise.
As I learn more, I'll keep replacing this with my latest and greatest. Last update: 2/16/10.
Temperature conversion table generator

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  <title>Temperature conversion table generator</title>
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    tr {background-color:#FF8888;}
    td {padding-left:4; padding-right:4}
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<p>I'm currently learning PHP, too. This is just my latest 
    mini-exercise.</p>

<p>As I learn more, I'll keep replacing this with my latest and 
    greatest. Last update: 2/16/10.</p>

<h2>Temperature conversion table generator</h2> 

<?php
set_time_limit(1);
error_reporting(E_ALL);

function main() {
  $lowF = get_param("lowF");
  $highF = get_param("highF");
  $stepF = get_param("stepF");
  generate_form($lowF, $highF, $stepF);
  if (get_param("Submit")
      && is_valid_request($lowF, $highF, $stepF))
        generate_table($lowF, $highF, $stepF);
}

function get_param($var_name) {
  if (isset($_REQUEST[$var_name]))
    return $_REQUEST[$var_name];
  return "";
}

function generate_form($lowF, $highF, $stepF) {
    $script = basename(__FILE__); 
    echo <<< _END
    <form action="$script" method="post"> 
       Starting temperature (°F): 
	 <input type="text" name="lowF" value="$lowF"><br /> 
       Ending temperature (°F): 
	 <input type="text" name="highF" value="$highF"><br /> 
       Step size: <input type="text" name="stepF" value="$stepF"><br /> 
       <input type="submit" name="Submit" value="Generate table"><br /> 
    </form>\n\n
_END;
}

function is_valid_request($lowF, $highF, $stepF) {
  $warning = "";

  if (!is_numeric($lowF) || !is_numeric($highF) || !is_numeric($stepF))
    $warning = "Please enter a number in each box.";
  elseif ($lowF >= $highF) 
    $warning = "Start temp must be less than end temp.";
  elseif ($stepF <= 0)
    $warning = "Step size must be positive.";
  elseif (($highF - $lowF) / $stepF >= 1000) 
    // If exactly equal to 1000, that's 1001 rows!
    $warning = "Table would have over 1000 rows. Please enter a "
		. "larger step size or a smaller range.";

  if ($warning)
    echo "<p style=\"color:red\">$warning</p>";
    
  return (!$warning);
}

function generate_table($lowF, $highF, $stepF) {
  echo "<table border=\"0\">\n";
  echo "  <tr style=\"background-color:#8888FF\">\n";
  echo "    <th>°F</th><th>°C</th>\n";
  echo "  </tr>\n\n";

  $lowC = ($lowF - 32) * 5 / 9;
  $stepC = $stepF * 5 / 9;

  for ($tempF = $lowF, $tempC = $lowC; 
       $tempF <= $highF; 
       $tempF += $stepF, $tempC += $stepC
      ) {
    echo "  <tr><td>" . round($tempF, 4) . "</td>" .
         "<td>" . round($tempC, 1) . "</td></tr>\n";
  }
  echo "</table>\n";
}

main();
?>

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