Simple PHP email test script.

mail.php

<?php
// Disclaimer: this script is for test purposes only, it is not intended to be secure.
ini_set('display_errors', 1);
error_reporting(E_ALL);
if (isset($_POST['to']) &&
        isset($_POST['from']) &&
        isset($_POST['subject']) &&
        isset($_POST['message'])) {
    $headers = 'From: ' . $_POST['from'] . PHP_EOL
             . 'Reply-To: ' . $_POST['from'] . PHP_EOL
             . 'X-Mailer: PHP/' . phpversion();
    if (mail($_POST['to'], $_POST['subject'], $_POST['message'], $headers)) {
        echo 'Mail function returned success.';
    } else {
        echo 'Mail function returned failure.';
    }
}
$version = phpversion();
$date = date('Y-m-d H:i:s');
$fullURL = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on' ? 'https' : 'http') . '://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
$from = 'php@' . $_SERVER['SERVER_NAME'];
?>
<style type="text/css">
    form {
        margin: auto;
        width: 600px;
    }
    input {
        width: 200px;
    }
    textarea {
        width: 340px;
        height: 200px;
    }
</style>
<form method="post" action="">
    <label>To<label><br/>
    <input type="text" name="to" value="" /><br/>
    <label>From<label><br/>
    <input type="text" name="from" value="<?= $from; ?>" /><br/>
    <label>Subject<label><br/>
    <input type="text" name="subject" value="Test Email" /><br/>
    <label>Message<label><br/>
    <textarea name="message">Hi, this is a quick test email from PHP <?= $version; ?> at <?= $fullURL; ?> on <?= $date; ?></textarea><br/>
    <button>Submit</button>
</form>