Information may be passed to functions via the argument list,
which is a comma-delimited list of expressions.
Making arguments be passed by reference
By default, function arguments are passed by value (so that if
the value of the argument within the function is changed, it does
not get changed outside of the function). To allow a function to modify its
arguments, they must be passed by reference.
To have an argument to a function always passed by reference, prepend an
ampersand (&) to the argument name in the function definition:
Example #2 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 $str; // outputs 'This is a string, and something extra.'
?>
Default argument values
A function may define C++-style default values for scalar
arguments as follows:
Example #3 Use of default parameters in functions
<?php
function makecoffee($type = "cappuccino")
{
return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>
The output from the above snippet is:
Making a cup of cappuccino.
Making a cup of .
Making a cup of espresso.
PHP also allows the use of arrays and the special type NULL
as default values, for example:
Example #4 Using non-scalar types as default values
<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
$device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>
The default value must be a constant expression, not (for
example) a variable, a class member or a function call.
Note that when using default arguments, any defaults should be on
the right side of any non-default arguments; otherwise, things
will not work as expected. Consider the following code snippet:
Example #5 Incorrect usage of default function arguments
<?php
function makeyogurt($type = "acidophilus", $flavour)
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // won't work as expected
?>
The output of the above example is:
Warning: Missing argument 2 in call to makeyogurt() in
/usr/local/etc/httpd/htdocs/phptest/functest.html on line 41
Making a bowl of raspberry .
Now, compare the above with this:
Example #6 Correct usage of default function arguments
<?php
function makeyogurt($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.
Variable-length argument lists
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.
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