Information may be passed to functions via the argument list,
which is a comma-delimited list of expressions.
PHP supports passing arguments by value (the default), passing by
reference, and default argument
values. Variable-length argument lists are supported only
in PHP 4 and later; see Variable-length argument
lists and the function references for
func_num_args(),
func_get_arg(), and
func_get_args() for more information. A
similar effect can be achieved in PHP 3 by passing an array of
arguments to a function:
Example 17-5. Passing arrays to functions
<?php
function takes_array($input)
{
echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>
|
|
By default, function arguments are passed by value (so that if
you change the value of the argument within the function, it does
not get changed outside of the function). If you wish to allow a
function to modify its arguments, you must pass them by
reference.
If you want an argument to a function to always be passed by
reference, you can prepend an ampersand (&) to the argument
name in the function definition:
Example 17-6. Passing function parameters by reference
<?php
function add_some_extra(&$string)
{
$string .= 'and something extra.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str; // outputs 'This is a string, and something extra.'
?>
|
|
A function may define C++-style default values for scalar
arguments as follows:
Example 17-7. Use of default parameters in functions
<?php
function makecoffee($type = "cappuccino")
{
return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee("espresso");
?>
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|
The output from the above snippet is:
Making a cup of cappuccino.
Making a cup of espresso. |
Also PHP allows you to use arrays and special type NULL as
default values, for example:
Example 17-8. Using non-scalar types as default values
<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
$device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>
|
|
The default value must be a constant expression, not (for
example) a variable, a class member or a function call.
Note that when using default arguments, any defaults should be on
the right side of any non-default arguments; otherwise, things
will not work as expected. Consider the following code snippet:
Example 17-9. Incorrect usage of default function arguments
<?php
function makeyogurt($type = "acidophilus", $flavour)
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // won't work as expected
?>
|
|
The output of the above example is:
Warning: Missing argument 2 in call to makeyogurt() in
/usr/local/etc/httpd/htdocs/php3test/functest.html on line 41
Making a bowl of raspberry . |
Now, compare the above with this:
Example 17-10. Correct usage of default function arguments
<?php
function makeyogurt($flavour, $type = "acidophilus")
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // works as expected
?>
|
|
The output of this example is:
Making a bowl of acidophilus raspberry. |
Note:
As of PHP 5, default values may be passed by reference.
PHP 4 and above has support for variable-length argument lists in
user-defined functions. This is really quite easy, using the
func_num_args(),
func_get_arg(), and
func_get_args() functions.
No special syntax is required, and argument lists may still be
explicitly provided with function definitions and will behave as
normal.
