<?php
$host = '127.0.0.1';
$db   = 'test';
$user = 'root';
$pass = '';
$charset = 'utf8';

$dsn = "mysql:host=$host;dbname=$db;charset=$charset";
$opt = [
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES   => false,
];
$pdo = new PDO($dsn, $user, $pass, $opt);

<?php
require_once "lib/nusoap.php";
require_once "conexion.php";

function muestraPersonas($pdo) {
    $consulta = $pdo->query("SELECT * FROM personas");
    return $consulta;
}

if(!isset($HTTP_RAW_POST_DATA)) {
$HTTP_RAW_POST_DATA = file_get_contents("php://input");
}
$server = new soap_server();
$server->register("muestraPersonas");
$server->service($HTTP_RAW_POST_DATA);

<?php
require_once "lib/nusoap.php";
require_once "conexion.php";

$cliente = new nusoap_client("http://localhost/WebService/soap_conexion.php");

$resultado = $cliente->call("muestraPersonas",$pdo);