PHP: Food Macro Percentage Calculator

Macro % Calculator

The code used for index.php is below:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="../styles.css">
    <title>PHP: Food Macro Percentage Calculator</title>
</head>
<body>
    <h1 class="lesson-h1">PHP: Food Macro Percentage Calculator</h1>
    <?php

        $error = "";
        $message = "";

        $calories = htmlentities($_POST['calories']);
        $fat = htmlentities($_POST['fat']);
        $carbs = htmlentities($_POST['carbs']);
        $protein = htmlentities($_POST['protein']);

        if(isset($_POST['calories'])){
    
            if(empty($_POST['calories']) || empty($_POST['fat']) || empty($_POST['carbs']) || empty($_POST['protein'])){
                $error = "<h3>Error #22: All Fields Are Required</h3>";
            } else {
                $fat_calories = $fat * 9;
                $carb_calories = $carbs * 4;
                $protein_calories = $protein * 4;

                $fat_percent = $fat_calories / $calories;
                $carb_percent = $carb_calories / $calories;
                $protein_percent = $protein_calories / $calories;

                $total_calories = $fat_calories + $carb_calories + $protein_calories;

                if($total_calories > $calories){
                    $error = "<h3>Error #11: Grams Provided Does Not Match Calorie Count</h3>";
                } else {
                    $message = "<h3 style='text-decoration: underline;'>Macro Percentages</h3>
                    <h4>Total Calories: $calories</h4><hr>
                    <h4>Grams of Fat: $fat</h4>
                    <h4>Fat Percentage: ".substr($fat_percent, 2, 2)."% | $fat_calories/$calories Total Calories</h4><hr>
                    <h4>Grams of Carbohydrates: $carbs</h4>
                    <h4>Carbohydrate Percentage: ".substr($carb_percent, 2, 2)."% | $carb_calories/$calories Total Calories</h4><hr>
                    <h4>Grams of Protein: $protein</h4>
                    <h4>Protein Percentage: ".substr($protein_percent, 2, 2)."% | $protein_calories/$calories Total Calories</h4><hr>";
                }  
            }
        }
    ?>

    <form action="index.php" method="post" class='lesson-form'>
        <h4>Macro % Calculator</h4>
        <label for="calories">
            <input type="number" name="calories" id="calories" placeholder="Calories per Serving">
        </label>
        <label for="fat">
            <input type="number" name="fat" id="fat" placeholder="Grams of Fat">
        </label>
        <label for="carbs">
            <input type="number" name="carbs" id="carbs" placeholder="Grams of Carbs">
        </label>
        <label for="protein">
            <input type="number" name="protein" id="protein" placeholder="Grams of Protein">
        </label>
        <input type="submit" value="Calculate">
    </form>

    <?php
        echo "<h3>$error</h3>";
        echo "<div>$message</div><hr>";

        include('../show_code.php');
        show_code('index.php');
    ?>
    
</body>
</html>