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

146 Chapter 5 Reusing Code and Writing Functions
One other reason you may see this error message is that the function you are calling
is part of a PHP extension that is not loaded. For example, if you try to use functions
from the gd (image manipulation) library and you have not installed gd, you will see this
message.
Understanding Case and Function Names
Note that calls to functions are not case sensitive, so calls to function_name(),
Function_Name(), or FUNCTION_NAME() are all valid and all have the same result.You are
free to capitalize in any way you find easy to read, but you should aim to be consistent.
The convention used in this book, and most other PHP documentation, is to use all
lowercase.
It is important to note that function names behave differently to variable names.
Variable names are case sensitive, so $Name and $name are two separate variables, but
Name() and name() are the same function.
Defining Your Own Functions
In the preceding chapters, you saw many examples using some of PHP’s built-in functions.
However, the real power of a programming language comes from being able to
create your own functions.
The functions built into PHP enable you to interact with files, use a database, create
graphics, and connect to other servers. However, in your career, you often may need to
do something that the language’s creators did not foresee.
Fortunately, you are not limited to using the built-in functions; you can write your
own to perform any task that you like.Your code will probably be a mixture of existing
functions combined with your own logic to perform a task for you. If you are writing a
block of code for a task that you are likely to want to reuse in a number of places in a
script or in a number of scripts, you would be wise to declare that block as a function.
Declaring a function allows you to use your own code in the same way as the built-in
functions.You simply call your function and provide it with the necessary parameters.This
means that you can call and reuse the same function many times throughout your script.
Examining Basic Function Structure
A function declaration creates or declares a new function.The declaration begins with the
keyword function, provides the function name and parameters required, and contains
the code that will be executed each time this function is called.
Here is the declaration of a trivial function:
function my_function() {
echo ‘My function was called’;
}