Take user information and check if domain is available

Question

I want to make a code where it checks the user’s information and sees if the domain exists. However, it returns absolutely nothing.

HTML:

 <form action="teste.php" method="POST" class="form-inline">
      <label class="sr-only" for="inlineFormInputName2">WWW</label>
            <input type="text" class="p-3 form-control form-control-lg mb-2 mr-sm-2 input-ro" id="inlineFormInputName2" placeholder="www." readonly>

      <label class="sr-only" for="inlineFormInputGroupUsername2">Domínio</label>
      <div class="input-group justify-content-center mb-2 mr-sm-2">
            <input type="text" name="dominio" class="input-dom form-control form-control-lg" id="inlineFormInputGroupUsername2" placeholder="Digite o domínio desejado">
      </div>

      <div class="form-check mb-2 mr-sm-2 select-padding">
           <select name="com" class="form-control form-control-lg">
                   <option>.com</option>
                   <option>.com.br</option>
                   <option>.net</option>
           </select>
       </div>
       <button type="submit" class="btn btn-dominio btn-outline-danger">
               <i class="fas fa-search"></i>
       </button>
  </form>

PHP:

 <?
require "registro-de-dominios.php";

$dominio = $_POST['dominio'];
function dominio_disponivel($dominio){
 if(checkdnsrr($dominio, 'ANY') && gethostbyname($dominio) != $dominio)
        echo "<p>Domínio <b>disponível.</b></p>";
    else
        echo "<p>Domínio <b>indisponível.</b></p>";
}
?>