delete php record

I do not understand why this code is not executed to delete a user by entering an email:

<div class="lista-usuarios">
<h2>Lista de usuarios: </h2> <br>

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    <input type="text" name="email" id="email">
    <button type="submit" name="borrar_usuario">Eliminar</button>
</form>

<?php

foreach ($usuarios as $value) { 
  if ($value === '[email protected]') {?>
     <li id=<?php echo $value; ?> class='usuario'> <?php echo $value ?> </li> <?php }else{ ?>
     <li id=<?php echo $value; ?> class='usuario'> <?php echo $value ?> 
     <a href="#" onclick='preguntar(<?php echo $value; ?>)'>Eliminar</a>
     </li>
<?php
}
}

$mysqli = new mysqli($host,$user,$pass,$db);

if (isset($_POST['borrar_usuario'])) {
    if (isset($_POST['email'])) {
        $sql_borrar = 'DELETE FROM usuarios where email='. $_POST['email'] .'';
        $mysqli->query($sql_borrar);
    }
}
?>
</div>