In PHP 5 you can use SoapClient on the WSDL to call the web service functions. For example:

$client = new SoapClient("some.wsdl");

and $client is now an object which has class methods as defined in some.wsdl. So if there was a method called getTime in the WSDL then you would just call:

$result = $client->getTime();

And the result of that would (obviously) be in the $result variable. You can use the __getFunctions method to return a list of all the available methods.

You can create a very simple JSON/JSONP API without libraries.

Here's a very basic example where you'd take an array of data, then JSON encode that array and returns the JSON, or JSONP, depending on the request:

$data = array();
//....do something to prepare your data....

if(isset($_GET['callback'])) {
    $callback = preg_replace("/[^A-Za-z0-9\.]/", '', $_GET['callback']);
    header("Content-type: application/javascript");
    echo $callback . "(" . json_encode($data) . ");";
}
else{
    header("Content-type: application/json");
    echo json_encode($data); //JSON
}

I developed a class that is the PHP native SoapServer class' REST equivalent.

You just include the RestServer.php file and then use it as follows.

class Hello
{
  public static function sayHello($name)
  {
    return "Hello, " . $name;
  }
}

$rest = new RestServer(Hello);
$rest->handle();

Then you can make calls from another language like this:

http://myserver.com/path/to/api?method=sayHello&name=World

(Note that it doesn't matter what order the params are provided in the query string. Also, the param key names as well as the method name are case-insensitive.)

Get it here.