Passing arguments 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 parameter name in the function definition:

<?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 constant expression as argument to parameter that expects to be passed by reference.

Default parameter values

A function may define default values for parameters using syntax similar to assigning a variable. The default is used only when the parameter's argument is not passed. Note that passing null does not assign the default value.

<?php
function makecoffee($type = "cappuccino")
{
    return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>

Making a cup of cappuccino.
Making a cup of .
Making a cup of espresso.

Default parameter values may be scalar values, arrays, the special type null, and as of PHP 8.1.0, objects using the new ClassName() syntax.

<?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");?>

Making a cup of cappuccino with hands.
Making a cup of cappuccino, lavazza with teapot.

<?php
class DefaultCoffeeMaker {
    public function brew() {
        return "Making coffee.\n";
    }
}
class FancyCoffeeMaker {
    public function brew() {
        return "Crafting a beautiful coffee just for you.\n";
    }
}
function makecoffee($coffeeMaker = new DefaultCoffeeMaker)
{
    return $coffeeMaker->brew();
}
echo makecoffee();
echo makecoffee(new FancyCoffeeMaker);
?>

Making coffee.
Crafting a beautiful coffee just for you.

The default value must be a constant expression, not (for example) a variable, a class member or a function call.

Note that any optional parameters should be specified after any required parameters, otherwise they cannot be omitted from calls. Consider the following example:

<?php
function makeyogurt($container = "bowl", $flavour)
{
    return "Making a $container of $flavour yogurt.\n";
}

echo makeyogurt("raspberry"); // "raspberry" is $container, not $flavour
?>

Fatal error: Uncaught ArgumentCountError: Too few arguments
 to function makeyogurt(), 1 passed in example.php on line 42

Now, compare the above with this:

<?php
function makeyogurt($flavour, $container = "bowl")
{
    return "Making a $container of $flavour yogurt.\n";
}

echo makeyogurt("raspberry"); // "raspberry" is $flavour
?>

Making a bowl of raspberry yogurt.

As of PHP 8.0.0, named arguments can be used to skip over multiple optional parameters.

<?php
function makeyogurt($container = "bowl", $flavour = "raspberry", $style = "Greek")
{
    return "Making a $container of $flavour $style yogurt.\n";
}

echo makeyogurt(style: "natural");
?>

Making a bowl of raspberry natural yogurt.

As of PHP 8.0.0, declaring mandatory parameters after optional parameters is deprecated. This can generally be resolved by dropping the default value, since it will never be used. One exception to this rule are parameters of the form Type $param = null, where the null default makes the type implicitly nullable. This usage is deprecated as of PHP 8.4.0, and an explicit nullable type should be used instead.

<?php

function foo($a = [], $b) {}     // Default not used; deprecated as of PHP 8.0.0
function foo($a, $b) {}          // Functionally equivalent, no deprecation notice

function bar(A $a = null, $b) {} // As of PHP 8.1.0, $a is implicitly required
                                 // (because it comes before the required one),
                                 // but implicitly nullable (deprecated as of PHP 8.4.0),
                                 // because the default parameter value is null
function bar(?A $a, $b) {}       // Recommended

?>

Note: As of PHP 7.1.0, omitting a parameter which does not specify a default throws an ArgumentCountError; in previous versions it raised a Warning.

Note: Parameters that expect the argument by reference may have a default value.

Variable-length argument lists

PHP has support for variable-length argument lists in user-defined functions by using the ... token.

Parameter 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:

<?php
function sum(...$numbers) {
    $acc = 0;
    foreach ($numbers as $n) {
        $acc += $n;
    }
    return $acc;
}

echo sum(1, 2, 3, 4);
?>

10

... can also be used when calling functions to unpack an array or Traversable variable or literal into the argument list:

<?php
function add($a, $b) {
    return $a + $b;
}

echo add(...[1, 2])."\n";

$a = [1, 2];
echo add(...$a);
?>

3
3

You may specify normal positional parameters 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.

<?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);
?>

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 (&).

Named Arguments

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.

<?php
myFunction(paramName: $value);
array_foobar(array: $value);

// NOT supported.
function_name($variableStoringParamName: $value);
?>

<?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.

<?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.

<?php
htmlspecialchars($string, double_encode: false);
// Same as
htmlspecialchars($string, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, 'UTF-8', false);
?>

Passing an argument to the same named parameter multiple times results in an Error exception.

<?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

?>

As of PHP 8.1.0, it is possible to use named arguments after unpacking the arguments. A named argument must not override an already unpacked argument.

<?php
function foo($a, $b, $c = 3, $d = 4) {
  return $a + $b + $c + $d;
}

var_dump(foo(...[1, 2], d: 40)); // 46
var_dump(foo(...['b' => 2, 'a' => 1], d: 40)); // 46

var_dump(foo(...[1, 2], b: 20)); // Fatal error. Named parameter $b overwrites previous argument
?>