php oop constructor - php

Question

OK. here is what I'm trying to do:
class Image{
public $_image;
public $_extension;
public $_mime;
public $_size;
public $_location;
public $_description;
public function __construct($image, $location){
$this->_image = $image;
$this->_location = $location;
$this->_extension = getExtension();
$this->_mime = getMime();
$this->_size = getSize();
}
private functions fallow.....
}
But I keep getting an internal server error when I try to run it. When I comment out the method calls it works. So the question is can I call methods from inside the constructor or am I doing something wrong with the methods.

Answer 1

Do your functions getExtension, getMime and getSize exist? Are they methods on this class? If they are methods, they need to be called with $this->... as in
$this->_extension = $this->getExtension();
If they are not methods, and are functions, you need to make sure the files that contain/define them are loaded before you run the constructor.

Answer 2

Well ..this fragment of code will work as expected:
class Foo
{
protected $secret = null;
public function __construct( $data )
{
$this->secret = $this->makeSecret($data);
}
public function makeSecret( $data )
{
return md5( $data );
}
}
$bar = new Foo( 'lorem ipsum' );
That is not a problem.
But you should know, that is considered to be a bad practice - to do computation/work in the constructor. It makes that class practically untestable. Instead, if you need to perform some computation before "releasing" the object to the rest of the code, you should use a factory. Something along the lines of :
class ImageFactory
{
public function build($image, $location)
{
$instance = new Image($image, $location);
$instance->prepare();
return $instance;
}
}
The class would need some changes:
class Image
{
protected $_image; // you were leaking abstraction
protected $_extension;
protected $_mime;
protected $_size;
protected $_location;
protected $_description;
public function __construct($image, $location)
{
$this->_image = $image;
$this->_location = $location;
}
public function prepare()
{
$this->_extension = $this->getExtension();
$this->_mime = $this->getMime();
$this->_size = $this->getSize();
}
private functions fallow.....
}
Now when you need to create new object you do:
$factory = new ImageFactory;
$image = $factory->build( $file, '/uploads/' );
Of course the instance of ImageFactory can be reusable, and if all your images use the same $location, then you would pass that variable to factory at the initialization. And the factory would be able to "remember it" and pass to all the images it creates:
$factory = new ImageFactory('/uploads/');
$img1 = $factory->build( $file );
$img2 = $factory->build( $something_else );
This is actually how one should deal with creating multiple objects, which all need access to same DB connection instance.

Answer 3

Yes, you can call methods from within the constructor. Remember that the __construct() magic method was implemented in PHP 5. Prior to that, you created a function named the same as your class which acted as your constructor so depending on your PHP version, that could be a problem.
Additionally, the function calls you are making, are they in the class or external? If they are inside the class you need to call them this way:
$this->_extension = $this->getExtension();

Answer 4

You didnt specified what error you are expiriencing clearly. But try calling you class methods even inside the class using this keyword, otherwise it would not work:
public function __construct($image, $location)
{
$this->_image = $image;
$this->_location = $location;
$this->_extension = $this->getExtension();
$this->_mime = $this->getMime();
$this->_size = $this->getSize();
}
Would be a better idea to post your code for the methods you wrote. There could be something wrong within them as well. Possibly forgetting to return a result or something...