index.php

<?php
$nomErr = $mailErr = $telErr = $subjectErr = $messageErr = $prenomErr = "";
$name = $email = $tel = $message = $subject = $firstName = "";
$success =true;
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  if (empty($_POST["user_name"])) {
    $nomErr = "Name is required";
    $success =false;
  } else {
    $name = $_POST["user_name"];
    if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
        $nomErr = "Only letters and white space allowed";
        $success =false;
    }
  }
  if (empty($_POST["user_first_name"])) {
    $prenomErr = "First name is required";
    $success =false;
  } else {
    $firstName = $_POST["user_first_name"];
    if (!preg_match("/^[a-zA-Z ]*$/",$firstName)) {
        $prenomErr = "Only letters and white space allowed";
        $success =false;
    }
  }
  if (empty($_POST["user_email"])) {
    $mailErr = "Email is required";
    $success =false;
  } else {
    $email = $_POST["user_email"];
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
    $mailErr = "Invalid email format"; 
    $success =false;
    }
    
  }
  if (empty($_POST["mail_subject"])) {
    $subjectErr = "Subject is required";
    $success =false;
  } else {
    $subject = $_POST["mail_subject"];
  }
  if (empty($_POST["user_message"])) {
    $messageErr = "message is required";
    $success =false;
  } else {
    $message = $_POST["user_message"];
  }
  if (empty($_POST["user_tel"])) {
    $telErr = "telephone is required";
    $success =false;
    
  } else {
    $tel = $_POST["user_tel"];
  }
  if($success == true){
      echo "Félicitation le message a bien été envoyé";
      $name = $email = $tel = $message = $subject = $firstName = "";
  } else {
      echo "Veuillez controler vos informations et valider de nouveau le formulaire";
  }
}
?>

<form action="index.php" method="post">
    <div>
        <label for="nom">Nom :</label>
        <input type="text" id="nom" name="user_name" value ="<?php echo $name?>" required>
        <span class="error">* <?php echo $nomErr;?></span>
    </div>
    <div>
        <label for="prenom">Prenom :</label>
        <input type="text" id="prenom" name="user_first_name" value ="<?php echo $firstName?>" required>
        <span class="error">* <?php echo $prenomErr;?></span>
    </div>
    <div>
        <label for="courriel">Courriel :</label>
        <input type="email" id="courriel" name="user_email" value ="<?php echo $email?>" required pattern ="[A-Za-z0-9](([_\.\-]?[a-zA-Z0-9]+)*)@([A-Za-z0-9]+)(([_\.\-]?[a-zA-Z0-9]+)*)\.([A-Za-z]{2,})">
        <span class="error">* <?php echo $mailErr;?></span>
    </div>
    <div>
        <label for="tel">Téléphone :</label>
        <input type="tel" name="user_tel" id="tel" value ="<?php echo $tel?>" required pattern ="(01|02|03|04|05|06|07|08|09)[ \.\-]?[0-9]{2}[ \.\-]?[0-9]{2}[ \.\-]?[0-9]{2}[ \.\-]?[0-9]{2}">
        <span class="error">* <?php echo $telErr;?></span>
    </div>
    <div>
        <label for="subject">Sujet :</label>
        <select name="mail_subject" id="subject" required>
            <option value="devis" 
            <?php if ($subject == "devis")
            {
                echo "selected";
            }?>>Devis</option>
            <option value="recla" 
            <?php if ($subject == "recla")
            {
                echo "selected";
            }?>>Réclamation</option>
            <option value="autre" 
            <?php if ($subject == "autre")
            {
                echo "selected";
            }?>>Autres demandes</option>
        </select>
        <span class="error">* <?php echo $subjectErr;?></span>
    </div>
    <div>
        <label for="message">Message :</label>
        <textarea id="message" name="user_message" required><?php echo $message?></textarea>
        <span class="error">* <?php echo $messageErr;?></span>
    </div>
    <div class="button">
        <button type="submit">Envoyer votre message</button>
    </div>
</form>