<?php
function get_client($headers) {
    if (isset($headers['HTTP_X_FORWARDED_FOR'])) {
        return explode(', ', $headers['HTTP_X_FORWARDED_FOR'])[0];
    }
    else {
        return $_SERVER['REMOTE_ADDR'];
    }
}

// $headers = getallheaders();
$headers = array('HTTP_X_FORWARDED_FOR' => 'client, proxy1, proxy2');
echo "Client 1: " . get_client($headers) . "\n";

$headers = array('HTTP_X_FORWARDED_FOR' => 'client');
echo "Client 2: " . get_client($headers) . "\n";

$headers = array();
echo "Client 3: " . get_client($headers) . "\n";
?>