Send Mail from Webhook with plain PHP

form-handler.php

<?php

// Check if the webhook should be fired.
if ( ! isset( $_GET['mailme'] ) ) {
	return;
}

// Check if there is POST request.
if ( empty( $_POST ) ) {
	return;
}

// iterate over $_POST request and build message.
$message = '';

foreach ( $_POST as $key => $value ) {
	$message .= '<p><b>' . htmlspecialchars( $key ) . '</b>: ' . htmlspecialchars( $value ) . '<p>' . PHP_EOL;
}

// Send mail with PHP.
mail("contact@mywebsite.com","A new submission", $message );