OOP stands for Object Oriented Programming. Procedural programming is a process where the program follows a linear set of instructions.

The advantages of OOP over procedural programming are:

• With the code being organised into objects / classes, it creates a code base that is easier to maintain and upgrade
• Having code broken into objects, that are organised according to their specific function, it allows teams of developers to work in tandem on bigger applications, particularly with the added use of Name spacing.
• Objects / classes can be easily reused in other projects
• It allows for the development of more complex applications that are at the same time flexible.
• It helps to facilitate programming paradigms such a MVC ( Model, View, Controller)

TDD stands for Test Driven Development. From what I understand this methodology involves, a process of creating a series of tests depicting different scenarios / user cases for a particular function to pass against.

The process continues with a cycle of refactoring code and running tests until, the fully fleshed out function has passed the test of all the defined user cases.

Clearly defining what the user requirements are that need to met. This would typically be done with the client or relevant stakeholders. Once this has been done, can we then move onto, planning the technical structure of the application / feature.

Not completely sure.

I have uploaded my example to this github repository. A full working version can be seen here: http://php-lamp-104185.nitrousapp.com:3000/justit-php-test/

Not sure.

Echo 1: 100

Echo 2: 200

class foo
{
  public function change1( $input )
  {
    $input = 200;
  }
  public function change2( $input )
  {
    $input->value = 200;
  }
}
class bar
{
  public $value;
}

$obj = new foo();
$var = new bar();

// Object variable set with a value of 100
$var->value = 100;

$obj->change1( $var->value );
echo $var->value;
// Value 100

$var->value = 100;
$obj->change2( $var );
echo $var->value;
// Value 200

An error. $multiply is instantiating a new object. run() is a static function. To enable the function output a result the public static function should be converted to a public function. The called using the ->. Code modified below.

class multiply
{
  private $multiplier;
  function __construct( $multiplier )
  {
    $this->multiplier = $multiplier;
  }
  public function run( $input )
  {
    return $input * $this->multiplier;
  }
}
$multiply = new multiply( 20 );
echo multiply->run( 20 );

The switch cases do not have any break; statements.

Again the switch cases do not have any break; statements. This will cause the switch statement to continue.