Function arguments

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- Function arguments

## Function arguments

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

    <?phpfunction 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

    <?phpfunction 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.){    // ...}?>

### 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 argument name in the function definition:

Example #3 Passing function parameters by reference

    <?phpfunction 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.

### Default argument values

A function may define default values for arguments using syntax similar to assigning a variable. The default is used only when the parameter is not specified; in particular, note that passing `null` does not assign the default value.

Example #4 Use of default parameters in functions

    <?phpfunction 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.

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.

Example #5 Using non-scalar types as default values

    <?phpfunction 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");?>

Example #6 Using objects as default values (as of PHP 8.1.0)

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

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

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

Example #7 Incorrect usage of default function arguments

    <?phpfunction makeyogurt($container = "bowl", $flavour){    return "Making a $container of $flavour yogurt.\n";} echo makeyogurt("raspberry"); // "raspberry" is $container, not $flavour?>

The above example will output:

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

Now, compare the above with this:

Example #8 Correct usage of default function arguments

    <?phpfunction makeyogurt($flavour, $container = "bowl"){    return "Making a $container of $flavour yogurt.\n";} echo makeyogurt("raspberry"); // "raspberry" is $flavour?>

The above example will output:

    Making a bowl of raspberry yogurt.

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

Example #9 Correct usage of default function arguments

    <?phpfunction makeyogurt($container = "bowl", $flavour = "raspberry", $style = "Greek"){    return "Making a $container of $flavour $style yogurt.\n";}echo makeyogurt(style: "natural");?>

The above example will output:

    Making a bowl of raspberry natural yogurt.

As of PHP 8.0.0, declaring mandatory arguments after optional arguments is deprecated. This can generally be resolved by dropping the default value, since it will never be used. 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 #10 Declaring optional arguments after mandatory arguments

    <?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) {} // Still allowed; $a is required but nullable 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: Arguments that are passed 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.

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:

Example #11 Using `...` to access variable arguments

    <?phpfunction 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 #12 Using `...` to provide arguments

    <?phpfunction 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 #13 Type declared variable arguments

    <?phpfunction 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 (`&`).

### 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.

Example #14 Named argument syntax

    <?phpmyFunction(paramName: $value);array_foobar(array: $value);// NOT supported.function_name($variableStoringParamName: $value);?>

Example #15 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 #16 Same example as above with a different order of parameters

    <?phparray_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 #17 Combining named arguments with positional arguments

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

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

Example #18 Error thrown when passing the same parameter multiple times

    <?phpfunction foo($param) { ... }foo(param: 1, param: 2);// Error: Named parameter $param overwrites previous argumentfoo(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.

Example #19 Use named arguments after unpacking

    <?phpfunction foo($a, $b, $c = 3, $d = 4) {  return $a + $b + $c + $d;}var_dump(foo(...[1, 2], d: 40)); // 46var_dump(foo(...['b' => 2, 'a' => 1], d: 40)); // 46var_dump(foo(...[1, 2], b: 20)); // Fatal error. Named parameter $b overwrites previous argument?>