Object-Oriented-PHP - Classes

A class provides the definition of an object and it serves as a template for creating new objects.

Defining classes

To get a list of all the available classes, PHP has the function: get_declared_classes().

$classes= get_declared_classes();
print_r($classes);

To define a new class:

class SimpleClass {

// code goes here...

}

To check if a class exists there is the function class_exists, that receives a string and returns a boolean.

if (class_exists("SimpleClass")){
    echo "True";
} else{
    echo "False";
}
//true

Instances

Once a class has been declared we can call it by its name, and assign it to a variable. This process is called instantiation, where we instantiate an object from a class.

$obj= new SimpleClass;  //creates a new object from the class SimpleClass
echo get_class($obj);  // returns the name of the object class (SimpleClass)

Class Properties

Properties are variables, attributes that hold object values.

Define properties using var $VariableName

class SimpleClass {
   var $prop1 = "something";
   var $prop2 = 3;
}

To acess the properties:

$obj= new SimpleClass;
echo $obj->prop1;
//something
echo $obj->prop2;
//3

Functions for properties

get_class_var($string) - returns a list of properties defined in the class:

print_r(get_class_vars("SimpleClass"));
//Array ( [prop1] => something [prop2] => 3 )

get_object_vars($object) - returns a list of properties assigned to the object:

print_r(get_object_vars($obj));
//Array ( [prop1] => something [prop2] => 3 )

property_exists($mixed, $string) - checks if the object or class has a property.

if (property_exists("SimpleClass", "prop2")){
    echo "True";
} else{
    echo "False";
}
//true

Class methods

Methods are functions inside classes to perform object actions.

class SimpleClass {
   //properties
   var $prop1 = "something";
   var $prop2 = 3;
   //method
   function say_hello(){  
      return "Hello world!";
      }
}

To execute the method:

$obj= new SimpleClass;
echo $obj->say_hello();
//Hello world!

Refer to an instance

If we try to access the properties inside the function, it will not work.

class SimpleClass {
   //properties
   var $prop1 = "something";
   var $prop2 = 3;
   //method
   function say_hello(){  
      return $prop1;
      }
}
$obj= new SimpleClass;
echo $obj->say_hello();
//Undefined variable: prop1

To solve this problem we need to use the variable this:

class SimpleClass {
   var $prop1 = "something";
   var $prop2 = 3;
   function say_hello(){  
      return $this->prop1;
      }
}

$obj= new SimpleClass;
echo $obj->say_hello();
//something