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Friday, May 23, 2008

Object Oriented PHP...Creating and Using Classes

Object Oriented PHP…Creating and Using Classes

[Please Read Introduction to Classes in C++, Unit Conversion Program using Classes, A Multi-Purpose String Class in C++ etc. for more information. Despite being from C++'s perspective they're useful.]

In PHP a class is defined as following:

class myclass
{
    ...
    ...
}

For C/C++ programmers, do note that there’s no ‘;’ after the closing curly brace.

A variable can be defined inside a class (AKA member variable) as:

class myclass
{
    
//notice the use of 'var'
    
var $var;
    ...
    ...
}

NOTE: We have to use ‘var’ keyword to define variables.

A function can also be defined inside a class as following. These functions are known as Member Functions.

class myclass
{
    function 
myfunc()
    {
        echo 
"myfunc";
    }
    ...
    ...
}

Accessing Members

Classes as you might know is just like a user-defined data-type, they don’t have any useful existence before we create instances or objects of that class.

Objects of a class may be created as:

$ob1=new myclass;

Now that ‘$ob1’ is an object of ‘myclass’ it holds its copy of the member variables and functions declared in the class that can be accessed as:

$ob1->var;

And functions as:

$ob1->myfunc();

Consider the following example code:

class myclass
{
    var 
$var;

    function 
myfunc()
    {
        echo 
"myfunc";
    }
}
$ob1=new myclass;
$ob2=new myclass;

$ob2->var=10;

echo 
$ob1->var;
$ob1->myfunc();
$ob2->myfunc();

Access Specifiers

You may control the accessibility of member variables and member functions from outside the class using ‘Access Specifiers’. You have previously seen that we can access members of a class using the following form:

echo $ob1->var;

And the members were accessible, because that is the default Access Specified for all the members declared inside a class. So by default members of a class have been given ‘public’ access from outside the class which can also optionally be done using the ‘public’ keyword. Other access specifies include ‘private’ and ‘protected’. Privately declared members of a class are not accessible from outside the class (using the ‘->’ method we have discussed). They however are accessible only to other member functions of the same class. We’ll discuss about ‘protected’ access specifier in future posts as it first requires ‘inheritance’ to be discussed.

The following code illustrates it quite well:

class myclass2
{
    
//no 'var' keyword
    
private $var;

    function 
myfunc()
    {
        echo 
"myfunc";
        
$var=2;
        
// 'var' accessible to Member Function
        
echo $var;
    }
}

$ob1=new myclass2;

//not accessible
echo $ob1->var;

$ob1->myfunc();

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