Creating PHP class instance with a string

I have two classes, class ClassOne { } and class ClassTwo {}. I am getting a string which can be either "One" or "Two".

Instead of using a long switch statement such as:

switch ($str) {
    case "One":
        return new ClassOne();
    case "Two":
        return new ClassTwo();
}

Is there a way I can create an instance using a string, i.e. new Class("Class" . $str);?

Yes, you can!

$str = 'One';
$class = 'Class'.$str;
$object = new $class();

When using namespaces, supply the fully qualified name:

$class = '\Foo\Bar\MyClass'; 
$instance = new $class();

Other cool stuff you can do in php are:
Variable variables:

$personCount = 123;
$varname = 'personCount';
echo $$varname; // echo's 123

And variable functions & methods.

$func = 'my_function';
$func('param1'); // calls my_function('param1');

$method = 'doStuff';
$object = new MyClass();
$object->$method(); // calls the MyClass->doStuff() method. 

You can simply use the following syntax to create a new class (this is handy if you're creating a factory):

$className = $whatever;
$object = new $className;

As an (exceptionally crude) example factory method:

public function &factory($className) {

    require_once($className . '.php');
    if(class_exists($className)) return new $className;

    die('Cannot create new "' . $className . '" class - includes not found or class unavailable.');
}

have a look at example 3 from http://www.php.net/manual/en/language.oop5.basic.php

$className = 'Foo';
$instance = new $className(); // Foo()

Lets say ClassOne is defined as:

public class ClassOne
{
    protected $arg1;
    protected $arg2;

    //Contructor
    public function __construct($arg1, $arg2)
    {
        $this->arg1 = $arg1;
        $this->arg2 = $arg2;
    }

    public function echoArgOne
    {
        echo $this->arg1;
    }

}

Using PHP Reflection;

$str = "One";
$className = "Class".$str;
$class = new \ReflectionClass($className);

Create a new Instance:

$instance = $class->newInstanceArgs(["Banana", "Apple")]);

Call a method:

$instance->echoArgOne();
//prints "Banana"

Use a variable as a method:

$method = "echoArgOne";
$instance->$method();

//prints "Banana"

Using Reflection instead of just using the raw string to create an object gives you better control over your object and easier testability (PHPUnit relies heavily on Reflection)