PHP - Where Is Anti-formatting Performed?

Where should the validation take place?  Several options include:

I can easily create the standalone Validator class to validate scenarios such as whether a property is provided and whether it meets certain rules, however, other scenarios such as whether a record exists is closely linked to the mapper.  Also, the entity does a good job confirming that the write properties are being provided.  How important is it to locate validation at one place?

Thanks

<?php
use \Psr\Http\Message\ServerRequestInterface as Request;
use \Psr\Http\Message\ResponseInterface as Response;

$c = new \Slim\Container();

$c['validator'] = function ($c) {};
$c['pdo'] = function ($c) {};

$c['resourceService'] = function ($c) {
    return new Resource\ResourceService(
        new Resource\ResourceMapper(
            $c['pdo'],
            $c['validator']
        ),
        $c['validator']
    );
};

$c['resourceResponder'] = function ($c) {};
$app = new \Slim\App($c);

$app->get('/someResource', function (Request $request, Response $response) {
    return $this->resourceResponder->index($response, $this->resourceService->index($request->getQueryParams()));
});
$app->get('/someResource/{id:[0-9]+}', function (Request $request, Response $response, $args) {
    return $this->resourceResponder->detail($response, $this->resourceService->read($args['id']));
});
$app->post('/someResource', function (Request $request, Response $response, $args) {
    return $this->resourceResponder->create($response, $this->resourceService->create($request->getParsedBody()));
});
$app->put('/someResource/{id:[0-9]+}', function (Request $request, Response $response, $args) {
    return $this->resourceResponder->update($response, $this->resourceService->update($args['id'], $request->getParsedBody()));
});
$app->delete('/someResource/{id:[0-9]+}', function (Request $request, Response $response, $args) {
    return $this->resourceResponder->delete($response, $this->resourceService->delete($args['id']));
});

$app->run();

class ResourceService
{
    protected $mapper, $validator;

    public function __construct(Mapper $mapper, Validator $validator) {
        $this->mapper = $mapper;
        $this->validator = $validator;
    }

    public function index(array $params=[]):array {
        $index = $this->mapper->index($params);
        return $index;
    }

    public function read(int $id):Entity {
        $entity = $this->mapper->read($id);
        return $entity;
    }

    public function create(array $params):int {
        $entity=$this->mapper->create($params);
        $id=$this->mapper->save($entity);
        return $id;
    }

    public function update(int $id, array $params):int {
        $this->update->update($id, $params);
        return $id;
    }

    public function delete(int $id):null {
        $this->mapper->delete($id);
    }

}

class ResourceMapper
{
    protected $pdo, $validator;

    public function __construct(\Pdo $pdo, Validator $validator) {
        $this->pdo = $pdo;
        $this->validator = $validator;
    }

    public function index(array $params=[]):array {
        //query DB and return an array of Resources
    }

    public function read(int $id):Entity {
        if(!$params=$this->queryDatabase($id)) {
            throw new \Exception("ID $id does not exist");
        }
        return new Resource($params);
    }

    public function create(array $params):int {
        //Or should the service create the entity?
        return new ResourceEntity($params);
    }

    public function save(Entity $entity):int {
        //Save the data.  What if a duplicate error?
        return $this->pdo->lastInsertId();
    }

    public function update(int $id, array $params):null {
        //update database.  What if id doesn't exist?
    }

    public function delete(int $id):null {
        //Delete from DB.  What if id doesn't exist or foreign key constraint?
    }

}

class ResourceEntity
{
    public function __construct(array $params, Validator $validator) {
        //As applicable
    }
}

Below is my contact from - and I have set anti spam question as I don't like captcha. How to I code the post/human bit so it is case insensitive?
I am a designer so I don't know what I am doing ☹️.

<div class="one-half-column-right" id="contactform">
 <form method="post" action="index.php#contactform">
<label>Name*</label>
<div class="clear"></div>
<input name="name" placeholder="Type Here">
 <label>Email*</label>
 <div class="clear"></div>
      <input name="email" type="email" placeholder="Type Here">
<label>Message</label>
<textarea name="message" placeholder="Type Here"></textarea>
 <label>*If today is Tuesday, what is tomorrow? <br> [lowercase answer please]<br> (Anti-spam)</label>
<input name="human" placeholder="Type Here">
<input id="submit" name="submit" type="submit" value="Submit">
</form>
 <?php
    $name = $_POST['name'];
    $email = $_POST['email'];
    $message = $_POST['message'];
    $from = 'From: Website Form'; 
    $to = ‘name@name.com’; 
    $subject = 'website form enquiry';
    $human = $_POST['human'];


$headers .= 'From: '.$from."\r\n".
    'Reply-To: '.$from."\r\n" .
    'X-Mailer: PHP/' . phpversion();
    $body = "From: $name\n E-Mail: $email\n Message:\n $message";
                
    if ($_POST['submit'] && $human == ‘wednesday’) {    
        if (mail ($to, $subject, $body, $from)) { 
        echo '<p style="font-family: Montserrat, Helvetica, Arial, sans-serif; font-weight: 600; text-align:center; font-size: 16px; color: #000; text-transform: uppercase; background-color: #FFD700"> 

     
      Request has been sent. We will get back to within 48 hours!<br></p>';

    } else { 
        echo '<p style="font-family: Montserrat, Helvetica, Arial, sans-serif; font-weight: 600; text-align:center; font-size: 16px; color: #000; text-transform: uppercase; background-color: #FFD700"> 

       Something went wrong, go back and try again!</p>'; 
    } 
    } else if ($_POST['submit'] && $human != '') {
    echo '<p style="font-family: Montserrat, Helvetica, Arial, sans-serif; font-weight: 600; text-align:center; font-size: 16px; color: #000; text-transform: uppercase; background-color: #FFD700"> 
    You answered the anti-spam question incorrectly!</p>';
    }
?>
    <!--// form //-->