PHP - Validar campo select

<section>

    <div class="form-row">

        <div class="title">Tu nombre y apellido</div>

        <input type="text" name="name" data-displayname="Name" class="field" placeholder="Tu nombre y apellido" required>

    </div>

    <div class="form-row">

        <div class="title">Tu e-mail</div>

        <input type="email" name="email" data-displayname="Email" class="field" placeholder="Tu e-mail" required>

    </div>

    <div class="form-row">

        <div class="title">Tu teléfono</div>

        <input type="tel" name="phone" data-displayname="Phone" class="field" placeholder="Tu teléfono" required>

    </div>

    <div class="form-row">

        <div class="title">Tema</div>

        <select multiple name="subject" data-displayname="Subject" class="field" placeholder="Seleciona una opción" required>

            <option default>Seleccione una opción</option>

            <option value="colegioBelgrano">Colegio Belgrano</option>

            <option value="colegioUrquiza">Colegio Urquiza</option>

            <option value="colegioSarmiento">Colegio Sarmiento</option>

            <option value="escueelaTecnica">Escuela Técnica</option>

            <option value="escuelaNocturna">Escuela Nocturna</option>

        </select>

<?php

// Init the Custom Configs

require_once 'configs.php';

$fields = array();

$fields[] = array('name' => 'name',			'display_name' => 'Name',           'type' => 'text',          'required' => true);

$fields[] = array('name' => 'email',		'display_name' => 'Email',          'type' => 'email',         'required' => true,        'is_user_email' => true);

$fields[] = array('name' => 'phone',		'display_name' => 'Phone',          'type' => 'tel',           'required' => true);

$fields[] = array('name' => 'subject',		'display_name' => 'Subject',        'type' => 'select',        'required' => true);

$fields[] = array('name' => 'message',		'display_name' => 'Message',        'type' => 'textarea',      'required' => true);

$fields[] = array('name' => 'files',		'display_name' => 'Upload file',    'type' => 'file',          'required' => false);

$fields[] = array('name' => 'option',		'display_name' => 'Option',         'type' => 'radio',         'required' => true);

// Init the functions

require_once 'functions.php';

// Custom Overrides before processing request

function beforeProcessing()

{

	global $lang;

	// Put here some language messages to override, this is useful if you need different messages only for this form

	// Example: $lang->emails_info->subject = "New subject for this form";

}

// Start processing

require_once 'process-upload.php';

?>

$_POST["subject"]