PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

166 Chapter 6 Object-Oriented PHP
How then do they work? If you instantiate the class
$a = new classname();
you can then use the __get() and __set() functions to check and set the value of any
attributes.
If you type
$a->$attribute = 5;
this statement implicitly calls the __set() function with the value of $name set to
“attribute”, and the value of $value set to 5.You need to write the __set() function
to do any error checking you want.
The __get() function works in a similar way. If, in your code, you reference
$a->attribute
this expression implicitly calls the __get() function with the parameter $name set to
“attribute”. It is up to you to write the __get() function to return the value.
At first glance, this code might seem to add little or no value. In its present form, this
is probably true, but the reason for providing accessor functions is simple:You then have
only one section of code that accesses that particular attribute.
With only a single access point, you can implement validity checks to make sure that
only sensible data is being stored. If it occurs to you later that the value of $attribute
should only be between 0 and 100, you can add a few lines of code once and check
before allowing changes.You could change the __set() function to look as follows:
function _set ($name, $value)
{
if( ($name="attribute") && ($value >= 0) && ($value attribute = $value;
}
With only a single access point, you are free to change the underlying implementation.
If, for some reason, you choose to change the way $attribute is stored, accessor functions
allow you to do this and change the code in only one place.
You might decide that, instead of storing $attribute as a variable, you will retrieve it
from a database only when needed, calculate an up-to-date value every time it is
requested, infer a value from the values of other attributes, or encode the data as a smaller
data type.Whatever change you decide to make, you can simply modify the accessor
functions. Other sections of code will not be affected as long as you make the accessor
functions still accept or return the data that other parts of the program expect.
Controlling Access with private and public
PHP uses access modifiers.They control the visibility of attributes and methods, and are
placed in front of attribute and method declarations. PHP supports the following three
different access modifiers: