Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right, before the function is actually called (eager evaluation).
PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists and Named Arguments are also supported.
Example #1 Passing arrays to functions
<?php
function takes_array($input)
{
echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>
As of PHP 8.0.0, the list of function arguments may include a trailing comma, which will be ignored. That is particularly useful in cases where the list of arguments is long or contains long variable names, making it convenient to list arguments vertically.
Example #2 Function Argument List with trailing Comma
<?php
function takes_many_args(
$first_arg,
$second_arg,
$a_very_long_argument_name,
$arg_with_default = 5,
$again = 'a default string', // This trailing comma was not permitted before 8.0.0.
)
{
// ...
}
?>
As of PHP 8.0.0, declaring mandatory arguments after optional arguments
is deprecated. This can generally be resolved by dropping the default value.
One exception to this rule are arguments of the form
Type $param = null, where the null default makes the type implicitly
nullable. This usage remains allowed, though it is recommended to use an
explicit nullable type instead.
Example #3 Declaring optional arguments after mandatory arguments
<?php
function foo($a = [], $b) {} // Before
function foo($a, $b) {} // After
function bar(A $a = null, $b) {} // Still allowed
function bar(?A $a, $b) {} // Recommended
?>
By default, function arguments are passed by value (so that if the value of the argument within the function is changed, it does not get changed outside of the function). To allow a function to modify its arguments, they must be passed by reference.
To have an argument to a function always passed by reference, prepend an ampersand (&) to the argument name in the function definition:
Example #4 Passing function parameters by reference
<?php
function add_some_extra(&$string)
{
$string .= 'and something extra.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str; // outputs 'This is a string, and something extra.'
?>
It is an error to pass a value as argument which is supposed to be passed by reference.
A function may define C++-style default values for scalar arguments as follows:
Example #5 Use of default parameters in functions
<?php
function makecoffee($type = "cappuccino")
{
return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>
The above example will output:
Making a cup of cappuccino. Making a cup of . Making a cup of espresso.
PHP also allows the use of arrays and the special type null
as default values, for example:
Example #6 Using non-scalar types as default values
<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
$device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>
The default value must be a constant expression, not (for example) a variable, a class member or a function call.
Note that when using default arguments, any defaults should be on the right side of any non-default arguments; otherwise, things will not work as expected. Consider the following code snippet:
Example #7 Incorrect usage of default function arguments
<?php
function makeyogurt($type = "acidophilus", $flavour)
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // won't work as expected
?>
The above example will output:
Warning: Missing argument 2 in call to makeyogurt() in /usr/local/etc/httpd/htdocs/phptest/functest.html on line 41 Making a bowl of raspberry .
Now, compare the above with this:
Example #8 Correct usage of default function arguments
<?php
function makeyogurt($flavour, $type = "acidophilus")
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // works as expected
?>
The above example will output:
Making a bowl of acidophilus raspberry.
Note: Arguments that are passed by reference may have a default value.
PHP has support for variable-length argument lists in
user-defined functions by using the
... token.
Note: It is also possible to achieve variable-length arguments by using func_num_args(), func_get_arg(), and func_get_args() functions. This technique is not recommended as it was used prior to the introduction of the
...token.
Argument lists may include the
... token to denote that the function accepts a
variable number of arguments. The arguments will be passed into the
given variable as an array; for example:
Example #9 Using ... to access variable arguments
<?php
function sum(...$numbers) {
$acc = 0;
foreach ($numbers as $n) {
$acc += $n;
}
return $acc;
}
echo sum(1, 2, 3, 4);
?>
The above example will output:
10
... can also be used when calling functions to unpack
an array or Traversable variable or
literal into the argument list:
Example #10 Using ... to provide arguments
<?php
function add($a, $b) {
return $a + $b;
}
echo add(...[1, 2])."\n";
$a = [1, 2];
echo add(...$a);
?>
The above example will output:
3 3
You may specify normal positional arguments before the
... token. In this case, only the trailing arguments
that don't match a positional argument will be added to the array
generated by ....
It is also possible to add a
type declaration before the
... token. If this is present, then all arguments
captured by ... must match that parameter type.
Example #11 Type declared variable arguments
<?php
function total_intervals($unit, DateInterval ...$intervals) {
$time = 0;
foreach ($intervals as $interval) {
$time += $interval->$unit;
}
return $time;
}
$a = new DateInterval('P1D');
$b = new DateInterval('P2D');
echo total_intervals('d', $a, $b).' days';
// This will fail, since null isn't a DateInterval object.
echo total_intervals('d', null);
?>
The above example will output:
3 days Catchable fatal error: Argument 2 passed to total_intervals() must be an instance of DateInterval, null given, called in - on line 14 and defined in - on line 2
Finally, variable arguments can also be passed
by reference by
prefixing the ... with an ampersand
(&).
No special syntax is required to note that a function is variadic; however access to the function's arguments must use func_num_args(), func_get_arg() and func_get_args().
The first example above would be implemented as follows in old versions of PHP:
Example #12 Accessing variable arguments in old PHP versions
<?php
function sum() {
$acc = 0;
foreach (func_get_args() as $n) {
$acc += $n;
}
return $acc;
}
echo sum(1, 2, 3, 4);
?>
The above example will output:
10
PHP 8.0.0 introduced named arguments as an extension of the existing positional parameters. Named arguments allow passing arguments to a function based on the parameter name, rather than the parameter position. This makes the meaning of the argument self-documenting, makes the arguments order-independent and allows skipping default values arbitrarily.
Named arguments are passed by prefixing the value with the parameter name followed by a colon. Using reserved keywords as parameter names is allowed. The parameter name must be an identifier, specifying dynamically is not allowed.
Example #13 Named argument syntax
<?php
myFunction(paramName: $value);
array_foobar(array: $value);
// NOT supported.
function_name($variableStoringParamName: $value);
?>
Example #14 Positional arguments versus named arguments
<?php
// Using positional arguments:
array_fill(0, 100, 50);
// Using named arguments:
array_fill(start_index: 0, count: 100, value: 50);
?>
The order in which the named arguments are passed does not matter.
Example #15 Same example as above with a different order of parameters
<?php
array_fill(value: 50, count: 100, start_index: 0);
?>
Named arguments can be combined with positional arguments. In this case, the named arguments must come after the positional arguments. It is also possible to specify only some of the optional arguments of a function, regardless of their order.
Example #16 Combining named arguments with positional arguments
<?php
htmlspecialchars($string, double_encode: false);
// Same as
htmlspecialchars($string, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, 'UTF-8', false);
?>
Passing the same parameter multiple times results in an Error exception.
Example #17 Error exception when passing the same parameter multiple times
<?php
function foo($param) { ... }
foo(param: 1, param: 2);
// Error: Named parameter $param overwrites previous argument
foo(1, param: 2);
// Error: Named parameter $param overwrites previous argument
?>