PHP - Validation form

I'm trying to create a simple validation script for a form but cannot get it to work properly. I only get redirected to an error page...

As you can see below only small and capitalized letters are allowed for first- and last name, only email structure for email and only numbers, spaces and (+) for the phone number. If the user input is not allowed the user gets redirected to a simple error page.

$first_name = $last_name = $email = $mobile = $country = "";

if (isset($_SERVER["REQUEST_METHOD"] == "POST")) {

    // Only small and capitalized letters allowed
    $first_name = test_input($_POST['first_name']);
    if(!preg_match("/^[a-zA-Z ]*$/",$first_name)) { 
        die("Error! Non allowed signs were used in 'first name'");
    }

    // Only small and capitalized letters allowed
    $last_name = test_input($_POST['last_name']);
    if(!preg_match("/^[a-zA-Z ]*$/",$last_name)) { 
        die("Error! Non allowed signs were used in 'last name'");
    }

    // Only email allowed
    $email = test_input($_POST['email']);
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        die ("Error! Non allowed signs were used in 'email'");
    }

    // Only numbers, space and + allowed
    $mobile = test_input($_POST['mobile']);
    if(!preg_match("/^[0-9 +-]+$/",$mobile)) { 
        die ("Error! Non allowed signs were used in 'mobile'");
    }

    // Country input (no validation)
    $country = $_POST['country'];

    }

    // Function test input
    function test_input($data) {
        $data = trim($data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        return $data;
    }

The HTML is basically this:

<form method="POST" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>"> 

    <input type="text" required="" name="first_name" style="width:100%" />

    [And so on...]

</form>