A simple program to ask the user to give a string or sentence and then the program will convert the vowel characters in a given string or sentence by the user using PHP programming language.

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Program Listing

<!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">
    <title>Capitalize Vowels in PHP</title>
    <style>
        *,html {
            box-sizing: border-box;
            padding: 0;
            margin: 0;
        }

        body {
            display: flex;
            justify-content: center;
            align-items: center;
            min-height: 100vh;
            background: #fefefe;
            font-family: sans-serif;
            background-color: #19BC9D;
        }

        .container {
            background: #fff;
            border: none;
            text-align: center;
            min-width: 300px;
            color: #fff;
        }

        .title {
            background-color: #14A083;
            padding: 20px;
            border: none;
        }

        .output_wrapper {
            padding: 20px;
            font-size: 20px;
            background-color: #fff;
            margin: 10px 0;
            border: none;
            color: #14A083;
        }

        h1 {
            margin: 0 0 5px;
            line-height: 1;
        }

        p {
            margin: 15px 0 0;
        }
        
        input {
            display: block;
            width: 100%;
            margin-bottom: 20px;
            padding: 10px;
            font-size: 15px;
            text-align: center;
        }

        .output {
            margin: 10px 0;
        }

        button,
        .btnRetry {
            background-color: #14A083;
            color: #fff;
            border: none;
            border-radius: 2px;
            text-align: center;
            text-transform: uppercase;
            text-decoration: none;
            cursor: pointer;
            font-size: 15px;
            padding: 15px 10px;
            width: 100%;
            display: block;
            margin: 20px 0 0;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="title">
            <h1>Capitalize Vowels in PHP</h1>
            <p>Created By:</p>
            <h2>&#187; Jake R. Pomperada, MAED-IT, MIT &#171;</h2>
        </div>
        <div class="output_wrapper">
        <?php
            if(isset($_POST["txtString"])){
                $string = strtolower($_POST["txtString"]);
                $vowels = array('a', 'e', 'i', 'o', 'u');
                $output = str_replace($vowels, array_map('strtoupper', $vowels), $string);

                echo '<p>The output is:</p>
                    <h3 class="output">'.$output.'</h3>
                    <a href="php_capitalize_vowels.php" class="btnRetry">Retry?</a>';
            } else {
                echo '<form action="" method="post">
                        <label>Enter a string here:</label>
                        <input type="text" name="txtString" required>

                        <button type="submit">Submit</button>
                    </form>';
            }
        ?>
        </div>
    </div>
</body>
</html>