Does not send mail php. What to do with it?

When customizing the feedback form in the mail, not sending in any way that I did. Tell me, please.

/* Set e-mail to Whom and from Whom to receive messages */ 
$to = ""; // Here you need to write an e-mail will be sent letters 
$from = ""; // Here you need to write an e-mail from whom to receive messages, such as no-reply(dog)

/* Specify variables to which to write the information from the form */
$first_name = $_POST['first_name'];
$email = $_POST['email'];
$phone = $_POST['phone'];
$message = $_POST['message'];
$subject = "Form submission messages from the site Cineholidays";

/* Variable that will be sent to your email with the values entered into fields */
$mail_to_myemail = "Hello! 
The message was sent from the site! 
Sender's name: $first_name
E-mail: $email
Phone number: $phone
Text message: $message
To reply to this email, create a new email, copy email address and paste in the to field."; 

$headers = "From: $from \r\n";

/* Send the message using mail () function */
mail($to, $subject, $mail_to_myemail, $headers . 'Content-type: text/plain; charset=utf-8')
 echo 'Sent';
else {
 echo 'Not sent';
echo "Message sent. Thank You." $first_name . "we'll contact You shortly.";
echo "<br /><br /><a href=''>return to the site.</a>";
<!--Redirect to home page after 3 seconds-->
<script language="JavaScript" type="text/javascript">
function changeurl(){eval(self.location="");}

And here is the code of my form:
<form action="sendemail.php" method="post" class="appointment-form ftco-animate">
 <div class="d-md-flex">
 <div class="form-group">
 <input type="text" class="form-control" name="first_name" placeholder="Your name">
 <div class="form-group ml-md-4">
 <input type="text" class="form-control" name="phone" placeholder="Your phone">
 <div class="d-md-flex">
 <div class="form-group">
 <textarea name="" id="" cols="30" rows="2" class="form-control" name="message" placeholder="message (optional)"></textarea>
 <div class="form-group ml-md-4">
 <input type="submit" name="submit" value="Send" class="btn btn-secondary py-3 px-4">
April 4th 20 at 13:06
1 answer
April 4th 20 at 13:08
Most often the problem is associated with your mail server settings on the hosting.
Try a few options of scripts to send if none works, write to support of hosting.

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