PHP - Validation form - php

Question

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>

Answer 1

if (isset($_SERVER["REQUEST_METHOD"] == "POST")) {`
is incorrect ~ isset returns a boolean which effectively makes this if(false=='POST') or similar
Try:
if ( $_SERVER["REQUEST_METHOD"] == "POST" ) {