PHP- Introduction to Object Oriented PHP

Introduction to Object-
Oriented PHP
2

• Topics:
o OOP concepts – overview, throughout the chapter
o Defining and using objects
• Defining and instantiating classes
• Defining and using variables, constants, and operations
• Getters and setters
o Defining and using inheritance and polymorphism
• Building subclasses and overriding operations
• Using interfaces
o Advanced object-oriented functionality in PHP
• Comparing objects, Printing objects,
• Type hinting, Cloning objects,
• Overloading methods, (some sections WILL NOT BE
COVERED!!!)
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Developing Object-Oriented PHP

• Object-oriented programming (OOP) refers to the creation of reusable
software object-types / classes that can be efficiently developed and
easily incorporated into multiple programs.
• In OOP an object represents an entity in the real world (a student, a desk,
a button, a file, a text input area, a loan, a web page, a shopping cart).
• An OOP program = a collection of objects that interact to solve a task /
problem.
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Object-Oriented Programming

• Objects are self-contained, with data and operations that pertain to them
assembled into a single entity.
o In procedural programming data and operations are separate → this
methodology requires sending data to methods!
• Objects have:
o Identity; ex: 2 “OK” buttons, same attributes → separate handle vars
o State → a set of attributes (aka member variables, properties, data
fields) = properties or variables that relate to / describe the object, with
their current values.
o Behavior → a set of operations (aka methods) = actions or functions
that the object can perform to modify itself – its state, or perform for
some external effect / result.
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• Encapsulation (aka data hiding) central in OOP
o = access to data within an object is available only via the object’s
operations (= known as the interface of the object)
o = internal aspects of objects are hidden, wrapped as a birthday
present is wrapped by colorful paper 
• Advantages:
o objects can be used as black-boxes, if their interface is known;
o implementation of an interface can be changed without a cascading
effect to other parts of the project → if the interface doesn’t change
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• Classes are constructs that define objects of the same type.
A class is a template or blueprint that defines what an object’s data and
methods will be.
Objects of a class have:
o Same operations, behaving the same way
o Same attributes representing the same features, but values of those
attributes (= state) can vary from object to object
• An object is an instance of a class.
(terms objects and instances are used interchangeably)
• Any number of instances of a class can be created.
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• Small Web projects
o Consist of web scripts designed and written using an ad-hoc approach; a
function-oriented, procedural methodology
• Large Web software projects
o Need a properly thought-out development methodology – OOP →
o OO approach can help manage project complexity, increase code
reusability, reduce costs.
o OO analysis and design process = decide what object types, what hidden
data/operations and wrapper operations for each object type
o UML – as tool in OO design, to allow to describe classes and class
relationships
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OOP in Web Programming

• A minimal class definition:
class classname { // classname is a PHP identifier!
// the class body = data & function member definitions
}
• Attributes
o are declared as variables within the class definition using keywords that
match their visibility: public, private, or protected.
(Recall that PHP doesn't otherwise have declarations of variables →
data member declarations against the nature of PHP?)
• Operations
o are created by declaring functions within the class definition.
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Creating Classes in PHP

• Constructor = function used to create an object of the
class
o Declared as a function with a special name:
function __construct (param_list) { … }
o Usually performs initialization tasks: e.g. sets attributes to appropriate
starting values
o Called automatically when an object is created
o A default no-argument constructor is provided by the compiler only if a
constructor function is not explicitly declared in the class
o Cannot be overloaded (= 2+ constructors for a class); if you need a
variable # of parameters, use flexible parameter lists…
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• Destructor = opposite of constructor
o Declared as a function with a special name, cannot take parameters
function __destruct () { … }
o Allows some functionality that will be automatically executed just before
an object is destroyed
 An object is removed when there is no reference variable/handle left
to it
 Usually during the "script shutdown phase", which is typically right
before the execution of the PHP script finishes
o A default destructor provided by the compiler only if a destructor
function is not explicitly declared in the class
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• Create an object of a class = a particular individual that is a member of the
class by using the new keyword:
$newClassVariable = new ClassName(actual_param_list);
• Notes:
o Scope for PHP classes is global (program script level), as it is for functions
o Class names are case insensitive as are functions
o PHP 5 allows you to define multiple classes in a single program script
o The PHP parser reads classes into memory immediately after functions ⇒
class construction does not fail because a class is not previously defined in
the program scope.
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Instantiating Classes

• From operations within the class, class’s data / methods can be
accessed / called by using:
o $this = a variable that refers to the current instance of the class,
and can be used only in the definition of the class, including the
constructor & destructor
o The pointer operator -> (similar to Java’s object member access operator
“.” )
o class Test {
public $attribute;
function f ($val) {
$this -> attribute = $val; // $this is mandatory!
} // if omitted, $attribute is
treated
} // as a local var in the function
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Using Data/Method Members
No $ sign here

• From outside the class, accessible (as determined by access modifiers) data
and methods are accessed through a variable holding an instance of the
class, by using the same pointer operator.
class Test {
public $attribute;
}
$t = new Test();
$t->attribute = “value”;
echo $t->attribute;
14
Using Data/Method Members

• Three access / visibility modifiers introduced in PHP 5, which affect the scope
of access to class variables and functions:
o public : public class variables and functions can be accessed from inside
and outside the class
o protected : hides a variable or function from direct external class access
+ protected members are available in subclasses
o private : hides a variable or function from direct external class access +
protected members are hidden (NOT available) from all subclasses
• An access modifier has to be provided for each class instance variable
• Static class variables and functions can be declared without an access
modifier → default is public
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Defining and Using Variables, Constants
and Functions

• Encapsulation : hide attributes from direct access from outside a class and
provide controlled access through accessor and mutator functions
o You can write custom getVariable() / setVariable($var) functions or
o Overload the functionality with the __get() and __set() functions in PHP
• __get() and __set()
o Prototype:
mixed __get($var);
// param represents the name of an attribute, __get returns the value of
that attribute
void __set($var, $value);
// params are the name of an attribute and the value to set it to
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Getters and Setters

• __get() and __set()
o Can only be used for non-static attributes!
o You do not directly call these functions;
For an instance $acc of the BankAccount class:
$acc->Balance = 1000;
implicitly calls the __set() function with the value of
$name set to ‘Balance’, and the value of $value set to
1000.
(__get() works in a similar way)
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Getters and Setters

• __get() and __set() functions’ value: a single access
point to an attribute ensures complete control over:
o attribute’s values
function __set($name, $value) {
echo "<p>Setter for $name called!</p>";
if (strcasecmp($name, "Balance")==0 && ($value>=0))
$this->$name = $value;
...
}
o underlying implementation: as a variable, retrieved from a
db when needed, a value inferred based on the values of
other attributes
→ transparent for clients as long as the accessor / mutator18
Getters and Setters

• Classes in Web development:
o Pages
o User-interface components
o Shopping carts
o Product categories
o Customers
• TLA Consulting example revisited - a Page class, goals:
o A consistent look and feel across the pages of the website
o Limit the amount of HTML needed to create a new page: easily
generate common parts, describe only uncommon parts
o Easy maintainable when changes in the common parts
o Flexible enough: ex. allow proper navigation elements in each page
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Designing Classes

• Attributes:
o $content → content of the page, a combination of HTML and text
o $title → page’s title, with a default title to avoid blank titles
o $keywords → a list of keywords, to be used by search engines
o $navigation → an associative array with keys the text for the buttons
and the value the URL of the target page
• Operations:
o __set()
o Display() → to display a page of HTML, calls other functions to display
parts of the page:
o DisplayTitle(), DisplayKeywords(), DisplayStyles(), DisplayHeader(),
DisplayMenu(), DisplayFooter() → can be overridden in a possible
subclass
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PHP- Introduction to Object Oriented PHP

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