The usefulness of
functions is greatly enhanced by the ability to pass information from the main
body of code to the function and in the opposite direction, or between one
function and another it is calling Values that are sent to a function are
called arguments.
Passing Argument to a
Function
To define arguments that
will be passed to a function, you provide a list of names for them between the
parentheses in the function statement, like this:
function Myfunction
($arg1, $arg2)
and to pass values to
the function as arguments by putting them between the parentheses following the
function's name in the statement that calls it, like this:
$answer = Myfunction (3,
5)
I've used actual number
here, but I could also have used variables. There is something to watch
out for with those variables! Remember from the
discussion about variables that
variables have a scope? This becomes very important here. The
function statement that defines a function is part of the function it defines;
therefore variable defined in that same statement are also part of the function
being defined; therefore their scope is local to the function. This means
that the variable's lifetime begins when the function starts to be processed and
ends when the function ends. Consequently, these variables are not
available in the main body of code (or in the calling function, if this function
is being called from within another function.) Simply using the same name
won't make it available! In fact, using the same name in the main body of
code will mean that you have two different variables with the same name.
This will not create an error, but it will certainly create a confusing program
to work on! It is best to use different names for the variables within
your functions (another "be your own best friend" tip!)
Returning Results
from a Function
A function returns a
result to the calling code by using the "return: keyword, like this:
return $result;
When the return
statement is processed, the function terminates and the value of the variable
specified is returned to the calling code. The return statement only
returns one variable -- if you need to return more than one value, you'd have to
define and return an array (we'll be getting into arrays a little later in this
series.)
You could have more than
one return statement in your function if, for example, you had conditional logic
(such as "if" statements) within your function, but only one of them will ever
be processed (since processing it terminates the function!)
An Example
Here's an example of a
couple of variables being passed to a function as arguments with a result
returned:
<html>
<head>
<title>Function
Arguments Example</title>
</head>
<body>
<h1>Showing a
Volume</h1>
<br />
<? php
function CalcVol ($x,
$y, $z)
{
$vol = $x * $y * $z;
return $vol;
}
$long = 3;
$wide = 4;
$high = 5;
print "The volume
of this object is " . CalcVol($long, $wide, $high) . " volumetric units.";
?>
</body>
</html>
In this example you can
see that a function called CalcVol is being defined, and that it receives three
parameters, $x, $y & $z. These are being used to specify the lengths of
the three axes of a solid object, which the function multiplies together to
determine the volume of the object. The function then returns the volume
to the calling code.
The function is being
called from the middle of a print statement, which passes the three values
contained in the variables $long, $wide & $high. Thus the print statement
will display the result of the calculation.
In the next part of this
tutorial series, we'll take a look and some functions that are built in to PHP
and at some more sophisticated ways to manipulate data being passed back and forth.
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