Sending email from forms via PHP not working. Code mistake or hosting side settings missing?

Question

I am trying to send an email after a form submission. The problem is that whenever I hit submit, the browser does take me to the form.php page but it says that "the website is currently unable to handle this request. HTTP ERROR 500". Here is my HTML and PHP. One thing to mention is that the HTML is on the contact.php file and the PHP is on the form.php file. Is there anything wrong with the code or implementation? Should I add any PHP code to the contact.php file? Should I do something extra on the hosting side?

HTML

<form action="form.php" method="POST"> 
        <label for="first_name">First name</label>
        <input type="text" name="first_name" required>

        <label for="last_name">Last Name</label>
        <input type="text" name="last_name" required>

        <label for="email">Email</label>
        <input type="email" name="email" required>

        <label for="phone">Phone</label>
        <input type="tel" name="phone" maxlength="10">

        <label for="comments">Comments</label>
        <input type="text" id="comments" name="comments">

        <input type="submit" id="submit">
    </form>

<?php
$firstName = $_POST["first_name"];
$lastName = $_POST["last_name"];
$email = $_POST["email]";
$phone = $_POST["phone"];
$comments = $_POST["comments"];

$email_from = "contact@mywebsite.com";

$to = "contact@mywebsite.com";

$email_subject = "New From Submission";

$message = "";
$message .= "First name: " .$firstName. "<br>";
$message .= "Last name: " .$lastName. "<br>";
$message .= "Email: " .$email. "<br>";
$message .= "Phone: " .$phone. "<br>";
$message .= "Comments: " .$comments. "<br>";
$headers = "From: $email_from \r\n\";

mail($to, $email_subject , $message, $headers);?>