What is the function __construct used for?

Question

__construct was introduced in PHP5 and it is the right way to define your, well, constructors (in PHP4 you used the name of the class for a constructor). You are not required to define a constructor in your class, but if you wish to pass any parameters on object construction then you need one.

An example could go like this:

class Database {
  protected $userName;
  protected $password;
  protected $dbName;

  public function __construct ( $UserName, $Password, $DbName ) {
    $this->userName = $UserName;
    $this->password = $Password;
    $this->dbName = $DbName;
  }
}

// and you would use this as:
$db = new Database ( 'user_name', 'password', 'database_name' );

Everything else is explained in the PHP manual: click here

__construct() is the method name for the constructor. The constructor is called on an object after it has been created, and is a good place to put initialisation code, etc.

class Person {

    public function __construct() {
        // Code called for each new Person we create
    }

}

$person = new Person();

A constructor can accept parameters in the normal manner, which are passed when the object is created, e.g.

class Person {

    public $name = '';

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

}

$person = new Person( "Joe" );
echo $person->name;

Unlike some other languages (e.g. Java), PHP doesn't support overloading the constructor (that is, having multiple constructors which accept different parameters). You can achieve this effect using static methods.

Note: I retrieved this from the log of the (at time of this writing) accepted answer.

Its another way to declare the constructor. You can also use the class name, for ex:

class Cat
{
    function Cat()
    {
        echo 'meow';
    }
}

and

class Cat
{
    function __construct()
    {
        echo 'meow';
    }
}

Are equivalent. They are called whenever a new instance of the class is created, in this case, they will be called with this line:

$cat = new Cat();

I think this is important to the understanding of the purpose of the constructor.
Even after reading the responses here it took me a few minutes to realise and here is the reason.
I have gotten into a habit of explicitly coding everything that is initiated or occurs. In other words this would be my cat class and how I would call it.

class_cat.php

class cat {
    function speak() {
        return "meow";  
    }
}

somepage.php

include('class_cat.php');
mycat = new cat;
$speak = cat->speak();
echo $speak;

Where in @Logan Serman's given "class cat" examples it is assumed that every time you create a new object of class "cat" you want the cat to "meow" rather than waiting for you to call the function to make it meow.

In this way my mind was thinking explicitly where the constructor method uses implicity and this made it hard to understand at first.

The constructor is a method which is automatically called on class instantiation. Which means the contents of a constructor are processed without separate method calls. The contents of a the class keyword parenthesis are passed to the constructor method.

The __construct method is used to pass in parameters when you first create an object--this is called 'defining a constructor method', and is a common thing to do.

However, constructors are optional--so if you don't want to pass any parameters at object construction time, you don't need it.

So:

// Create a new class, and include a __construct method
class Task {

    public $title;
    public $description;

    public function __construct($title, $description){
        $this->title = $title;
        $this->description = $description;
    }
}

// Create a new object, passing in a $title and $description
$task = new Task('Learn OOP','This is a description');

// Try it and see
var_dump($task->title, $task->description);

For more details on what a constructor is, see the manual.