Check if char is the same as alphabet letter condition regardless of uppercase and lowercase

Here's the sample code:
What I Want To Achieve: Regardless of it being uppercase and lowercase, if the user inputs a letter that's the same as the condition whether it's also uppercase and lowercase, I want it to be true. A = a, b = B.

Ex: (D or d == d) == true | Output: 3 - DEF (A or c == a or c) == true | Output: 2 - ABC

<!DOCTYPE html>
<html lang = "en">
<head>
  <title>Test</title>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <!-- BOOTSTRAP 5 -->
    <link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">
    <!-- jQuery -->
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js">
    </script>
    <!-- Sweet Alert 2 -->
    <script src="https://unpkg.com/sweetalert/dist/sweetalert.min.js">
    </script>
</head>

<body>

   <!-- Input and Output -->
            <!-- INPUT -->
            <div class="card">
                <div class="card-header"> Input Letter: </div>
                <form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
                    <div class="card-body">
                        <div class="row">
                            <div class="col-12">
                                <input type="text" name="telephoneCharacter" placeholder="Input letter." class="form-label input-responsive" maxlength = "1">
                            </div>
                            <div class="col-12">
                                <div class="col-auto d-flex justify-content-sm-end">
                                    <input type="submit" class="btn btn-dark">
                                </div>
                            </div>
                        </div>
                    </div>
                </form>
            </div>

            <!-- OUTPUT -->
            <div class="card-img-bottom d-flex justify-content-center">
                <div class="text-uppercase">
                    <p class="Display-1"> Output: </p>
                    <div class="container-fluid">
                        <?php
                        if ($_SERVER['REQUEST_METHOD'] == 'POST') {
                            $character = $_POST['telephoneCharacter'];

                            if ( ($character == 'A' || $character == 'a' ) || ($character == 'B' || $character == 'b' ) || ($character == 'C' || $character == 'c') ) {
                                print "<p class='text-muted h4'> 2 - ABC</p>";
                            } else if ( ($character == 'D' || $character == 'd' ) || ($character == 'E' || $character == 'e' ) || ($character == 'F' || $character == 'f') ) {
                                print "<p class='text-muted h4'> 3 - DEF</p>";
                            }  else if ( ($character == 'G' || $character == 'g' ) || ($character == 'H' || $character == 'h' ) || ($character == 'I' || $character == 'i') ) {
                                print "<p class='text-muted h4'> 4 - GHI</p>";
                            } else if ( ($character == 'J' || $character == 'j' ) || ($character == 'K' || $character == 'k' ) || ($character == 'L' || $character == 'l') ) {
                                print "<p class='text-muted h4'> 5 - JKL</p>";
                            } else if ( ($character == 'M' || $character == 'm' ) || ($character == 'N' || $character == 'n' ) || ($character == 'O' || $character == 'o') ) {
                                print "<p class='text-muted h4'> 6 - MNO</p>";
                            }                                         

                        }
                        ?>
                    </div>
                </div>
            </div>
            
            <!-- BOOTSTRAP 5 Script -->
                <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP JcXn/tWtIaxVXM" crossorigin="anonymous"></script>

</body>
</html>

CodePudding user response：

Create an array of valid characters all in the same case (upper in this case) and then compare the input, converted to upper with entries in the array

$character = 'd';

if ( in_array(strtoupper($character), ['A','B','C'] )) {
    print "<p class='text-muted h4'> 2 - ABC</p>";
}

if ( in_array(strtoupper($character), ['D','E','F'] )) {
    print "<p class='text-muted h4'> 3 - DEF</p>";
}
if ( in_array(strtoupper($character), ['G','H','I'] )) {
    print "<p class='text-muted h4'> 4 - GHI</p>";
}
if ( in_array(strtoupper($character), ['J','K','L'] )) {
    print "<p class='text-muted h4'> 5 - JKL</p>";
}
if ( in_array(strtoupper($character), ['M','N','O'] )) {
    print "<p class='text-muted h4'> 6 - MNO</p>";
}