What is the function __construct used for?

__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();

class Person{
 private $fname;
 private $lname;

 public function __construct($fname,$lname){
  $this->fname = $fname;
  $this->lname = $lname;
 }
}
$objPerson1 = new Person('john','smith');

__construct simply initiates a class. Suppose you have the following code;

Class Person { 

 function __construct() {
   echo 'Hello';
  }

}

$person = new Person();

//the result 'Hello' will be shown.

We did not create another function to echo the word 'Hello'. It simply shows that the keyword __construct is quite useful in initiating a class or an object.

A constructor allows you to initialize an object's properties upon creation of the object.

If you create a __construct() function, PHP will automatically call this function when you create an object from a class.

https://www.w3schools.com/php/php_oop_constructor.asp

Let me explain __construct() without first using the method ... One thing to know about __construct() is that it is an inbuilt function, well let me call it method in PHP. Just as we have print_r() for procedural, the __construct() is an inbuilt for OOP.

That being said, let's explore why you should use this function called __construct().

  /*=======Class without __construct()========*/
  class ThaddLawItSolution
   {
      public $description;
      public $url;
      public $ourServices;

      /*===Let us initialize a value to our property via the method set_name()==== */
     public function setName($anything,$anythingYouChoose,$anythingAgainYouChoose)
     {
      $this->description=$anything;
      $this->url=$anythingYouChoose;
      $this->ourServices=$anythingAgainYouChoose;
    }
    /*===Let us now display it on our browser peacefully without stress===*/
    public function displayOnBrowser()
    {
       echo "$this->description is a technological company in Nigeria and our domain name is actually $this->url.Please contact us today for our services:$this->ourServices";
    }
 
 }

         //Creating an object of the class ThaddLawItSolution
$project=new ThaddLawItSolution;
        //=======Assigning Values to those properties via the method created====//
$project->setName("Thaddlaw IT Solution", "https://www.thaddlaw.com", "Please view our website");
      //===========Let us now display it on the browser=======
$project->displayOnBrowser();

__construct() makes life for you very easy, imaging the time it took me to assigning values to those properties via that method. From the code above, I created an object which is first and then assign values to the properties which is second before finally showing it on the browser. But using __construct() while creating an object i.e. $project= new ThaddLawItSolution; you would do what you did for assigning values to that method immediately while creating the object, i.e.

$project=new ThaddLawItSolution("Thaddlaw IT Solution", "https://www.thaddlaw.com","Please view our website");

//===Let's now use __constructor=====

Just remove that method called setName and put __construct(); and when creating an object, you assign the values at once. That is the point behind the whole __construct() method. But note that this is an inbuilt method or function