Functions and Classes are language tools which help you organize the source code more efficiently, reduce repetition and generally make things less tangled and confusing.
These are language tools which are not necessary to complete your application, but you should really know that they exist and how to use them.
You have already met some functions in the previous examples – e.g.:
print_r – function to print an array
count – function to return the number of elements in an array
These are built-in PHP functions. PHP has many built-in functions, I will
show some of the very useful ones along the way. Apart from those
you can define your own functions. Here I define a function
which – well – makes Moo!:
Once the function is defined in the script, you can call it anywhere by writing:
We can define (and immediately call) a more complicated function
with the parameter
The above script will print:
Moo! Moo! Moo! Moo! Moo! Moo! Moo! Moo! Moo! Moo!
In the above example, the function
makeMoo now creates a string by
concatenating (with the
. operator) the individual Moos and returns
that string. The result string is printed when the function is called
There is a bit more you can do with function’s parameters. For example you can have a default values for them when you know that some function will be frequently called with particular values:
Or you can set a mandatory data-type for a function parameter – this functionality was extended in PHP7 to support scalar data-types too. Previous PHP versions could watch for classes and arrays only. This is useful when you write some code which is used by other programmers – you do not have to write that many lines of code to check what your function got from the outside world. A good IDE can also take advantage of the supplied information.
Classes are templates for Objects. Objects are structures which can contain fields (also called properties or attributes) with values (similar to associative arrays) and functions. In other words we say that Objects are instances of classes. Functions in classes and objects are called methods. Fields and methods are Class and Object members. Let’s see an example:
I defined a class named
Cow (it’s a nice convention to capitalize class names). The
class has one class member – the method makeMoo. Then I created an instance of
the Cow class – a cow
$betty. Once I have the cow object, I can call the
method to make Betty do Moo.
Notice, that you need to use the
-> operator to access object members. Each class member
has visibility – which can be either: public, protected, private:
Let’s make a more complicated class:
The above class, has three members – field
$name and methods
makeMoo. The method
__construct is a special method –
constructor and is called automatically when you create an instance of the
class. I’ve added a parameter
$name to the constructor so when I create an
instance of the class, I must pass a name of the cow
$name is marked as private, which means that it can be accessed only from
within the class itself. This is done using the statement
The special variable
$this refers to the current object and can be used only
inside object methods. The point of using private members is that their values
cannot be changed inadvertently.
If you have looked at the list of built-in PHP functions and classes
you may have wondered
what happens when you define a function or class which already exists. Well, a
conflict happens and it becomes unclear what function you’re calling. To solve this problem, PHP has
namespaces. Referring to classes in namespaces is done using the backslash
\ character, e.g.:
Built-in classes are in the root namespace:
$object = new \PDO();
Working with namespaces is a bit tricky, because it depends on whether and how your script uses them. At least you should be aware that namespaces exist and recognize namespaced classes.
Create the class
User, with the private fields
fields should be set through the constructor. The class should have the method
print the full user name. Create two users:
John Doe with email
Jane Dona with email
email@example.com. Print names of both users.
Now you should have basic understanding of what functions, classes and objects are. You should be able to call your own functions and built-in PHP functions. You should be able to define simple functions with parameters and return values. You should be able to create instances of classes and call methods.