how to create order using magento web services in php

You'll want to use the methods in the cart hierarchy to create a cart, add items to it, and then user the order method.

Here's the sample code the from API docs.

/**
 * Example of order creation
 * Preconditionsare as follows:
 * 1. Create a customer
 * 2. Сreate a simple product */

$user = 'apiUser';
$password = 'apiKey';
    $proxy = new SoapClient('http://magentohost/api/v2_soap/?wsdl');
    $sessionId = $proxy->login($user, $password);
    $cartId = $proxy->shoppingCartCreate($sessionId, 1);
    // load the customer list and select the first customer from the list
    $customerList = $proxy->customerCustomerList($sessionId, array());
    $customer = (array) $customerList[0];
    $customer['mode'] = 'customer';
    $proxy->shoppingCartCustomerSet($sessionId, $cartId, $customer);
    // load the product list and select the first product from the list
    $productList = $proxy->catalogProductList($sessionId);
    $product = (array) $productList[0];
    $product['qty'] = 1;
    $proxy->shoppingCartProductAdd($sessionId, $cartId, array($product));

    $address = array(
        array(
            'mode' => 'shipping',
            'firstname' => $customer['firstname'],
            'lastname' => $customer['lastname'],
            'street' => 'street address',
            'city' => 'city',
            'region' => 'region',
            'telephone' => 'phone number',
            'postcode' => 'postcode',
            'country_id' => 'country ID',
            'is_default_shipping' => 0,
            'is_default_billing' => 0
        ),
        array(
            'mode' => 'billing',
            'firstname' => $customer['firstname'],
            'lastname' => $customer['lastname'],
            'street' => 'street address',
            'city' => 'city',
            'region' => 'region',
            'telephone' => 'phone number',
            'postcode' => 'postcode',
            'country_id' => 'country ID',
            'is_default_shipping' => 0,
            'is_default_billing' => 0
        ),
    );
     // add customer address
    $proxy->shoppingCartCustomerAddresses($sessionId, $cartId, $address);
    // add shipping method
    $proxy->shoppingCartShippingMethod($sessionId, $cartId, 'flatrate_flatrate');

    $paymentMethod =  array(
        'po_number' => null,
        'method' => 'checkmo',
        'cc_cid' => null,
        'cc_owner' => null,
        'cc_number' => null,
        'cc_type' => null,
        'cc_exp_year' => null,
        'cc_exp_month' => null
    );
     // add payment method
    $proxy->shoppingCartPaymentMethod($sessionId, $cartId, $paymentMethod);
     // place the order
    $orderId = $proxy->shoppingCartOrder($sessionId, $cartId, null, null);

By default Magento does not allow you to create orders through the API.
Take a look in app/code/code/Mage/Sales/etc/wsdl.xml inside the tag <portType> and see what methods are supported for the Sales module.
In general you can only retrieve lists or individual objects, or add comments to them. For the an order you can change the status.
You are allowed to create shipments, invoices or credit memos, but not orders.
To be able to do it, you need to extend the wsdl and add your class with methods for creating an order.
No offense, but this might prove to be a big task for a beginner (even for someone with a little experience also).
If you want to give it a go read first how to extend the web service and how to override models in magento.
You can also take a look at this. It's a workaround on creating an order by going through the checkout process using the API. (I haven't tested it).
[EDIT]
I will leave this answer here as it serves as general knowledge, but I highly approve of Alan's answer. He really know what's his talking about. :)