[php] Setting public class variables

How do I set a public variable. Is this correct?:

class Testclass
{
  public $testvar = "default value";

  function dosomething()
  {
    echo $this->testvar;
  }
}

$Testclass = new Testclass();
$Testclass->testvar = "another value";    
$Testclass->dosomething();

For overloading you'd need a subclass:

class ChildTestclass extends Testclass {
    public $testvar = "newVal";
}

$obj = new ChildTestclass();
$obj->dosomething();

This code would echo newVal.

Inside class Testclass:

public function __construct($new_value)
{
    $this->testvar = $new_value;
}

You're "setting" the value of that variable/attribute. Not overriding or overloading it. Your code is very, very common and normal.

All of these terms ("set", "override", "overload") have specific meanings. Override and Overload are about polymorphism (subclassing).

From http://en.wikipedia.org/wiki/Object-oriented_programming :

Polymorphism allows the programmer to treat derived class members just like their parent class' members. More precisely, Polymorphism in object-oriented programming is the ability of objects belonging to different data types to respond to method calls of methods of the same name, each one according to an appropriate type-specific behavior. One method, or an operator such as +, -, or *, can be abstractly applied in many different situations. If a Dog is commanded to speak(), this may elicit a bark(). However, if a Pig is commanded to speak(), this may elicit an oink(). They both inherit speak() from Animal, but their derived class methods override the methods of the parent class; this is Overriding Polymorphism. Overloading Polymorphism is the use of one method signature, or one operator such as "+", to perform several different functions depending on the implementation. The "+" operator, for example, may be used to perform integer addition, float addition, list concatenation, or string concatenation. Any two subclasses of Number, such as Integer and Double, are expected to add together properly in an OOP language. The language must therefore overload the addition operator, "+", to work this way. This helps improve code readability. How this is implemented varies from language to language, but most OOP languages support at least some level of overloading polymorphism.

Use Constructors.

<?php
class TestClass
{
    public $testVar = "default value";
    public function __construct($varValue)
    {
       $this->testVar = $varValue;               
    }
}    
$object = new TestClass('another value');
print $object->testVar;
?>

class Testclass
{
  public $testvar;

  function dosomething()
  {
    echo $this->testvar;
  }
}

$Testclass = new Testclass();
$Testclass->testvar = "another value";    
$Testclass->dosomething(); ////It will print "another value"

Add getter and setter method to your class.

public function setValue($new_value)
{
    $this->testvar = $new_value;
}

public function getValue()
{
    return $this->testvar;        
}

If you are going to follow the examples given (using getter/setter or setting it in the constructor) change it to private since those are ways to control what is set in the variable.

It doesn't make sense to keep the property public with all those things added to the class.