validate email

contact.php

<?php

$DESTINATAIRE = 'mon@adresse.com';
$SUJET = 'Message reçu depuis '.$_SERVER['HTTP_HOST'].' ';

function valid_email($email) {
    
    $atom   = '[-a-z0-9!#$%&\'*+\\/=?^_`{|}~]';   
    $domain = '([a-z0-9]([-a-z0-9]*[a-z0-9]+)?)'; 
    $regex = '/^' . $atom . '+' .   
             '(\.' . $atom . '+)*' .
             '@'.
             '(' . $domain . '{1,63}\.)+' .  
             $domain . '{2,63}$/i';          
    return preg_match($regex, $email);

}


if( valid_email($_POST['email']) ) {

    $_POST = array_map('htmlentities',$_POST);
    
    $HTML = '
        <h3>' . $_POST['nom'] . ' ' . $_POST['prenom'] . ' ('. $_POST['email'] . ')</h3>
        <p>
        '.nl2br($_POST['message']).'
        </p>
    ';

    $ok = mail( $DESTINATAIRE,
                $SUJET,
                $HTML,
                "From:".trim($_POST['email'])."\r\n"
                ."Content-type:text/html;" );
    
    if($ok) header('location: succes.html') and exit();

}

header('location:?erreur') and exit();