Server-Side Scripting/Requests/PHP

index.php

<?php 

// Converts a Fahrenheit temperature to Celsius using a GET request and
// converts a Celsius temperature to Fahrenheit using a POST request.
//
// References:
//  https://www.mathsisfun.com/temperature-conversion.html
//  https://en.wikibooks.org/wiki/PHP_Programming
//  https://www.tutorialspoint.com/php/php_get_post.htm

$fahrenheit = process_fahrenheit();
$celsius = process_celsius();

function process_fahrenheit() {
    if (isset($_GET["fahrenheit"]) && is_numeric($_GET["fahrenheit"])) {
        $fahrenheit = $_GET["fahrenheit"];
        $celsius = ($fahrenheit - 32) * 5 / 9;
        $result = $fahrenheit . "° Fahrenheit is " .
            $celsius . "° Celsius";
    }
    else {
        $result = "";
    }

    return $result;
}

function process_celsius() {
    if (isset($_POST["celsius"]) && is_numeric($_POST["celsius"])) {
        $celsius = $_POST["celsius"];
        $fahrenheit = $celsius * 9 / 5 + 32;
        $result = $celsius . "° Celsius is " .
            $fahrenheit . "° Fahrenheit";
    }
    else {
        $result = "";
    }

    return $result;
}

?>

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Lesson 3</title>
    <link rel="stylesheet" href="styles.css">
    <style>
        p {
            min-height: 1em;
        }
    </style>
</head>
  <body>
    <form method="GET">
        <p>
            <label for="fahrenheit">Enter Fahrenheit temperature:</label>
            <input type="text" id="fahrenheit" name="fahrenheit">
            <input type="submit" value="Submit">
        </p>
        <p>
            <output><?=$fahrenheit?></output>
        </p>
    </form>
    <form method="POST">
        <p>
            <label for="celsius">Enter Celsius temperature:</label>
            <input type="text" id="celsius" name="celsius">
            <input type="submit" value="Submit">
        </p>
        <p>
            <output><?=$celsius?></output>
        </p>
    </form>
</body>
</html>
Try It

Copy and paste the code above into the following free online development environment or use your own PHP compiler / interpreter / IDE.