PHP - Arrow Functions

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Arrow functions were introduced in PHP 7.4 version. Arrow functions provide a simpler and more concise syntax for writing anonymous functions. With PHP 7.4, a keyword "fn" has been introduced for defining arrow functions, instead of the conventional use of the "function" keyword.

fn (argument_list) => expr

• There is only one expression after the "=>" symbol, and its value is the return value of the arrow function.

• The arrow function doesn’t have an explicit return statement.

• Like in the anonymous function, the arrow function is assigned to a variable for it to be called.

Example

The following example demonstrates how you can use the arrow function in PHP −

<?php
   $add = fn ($a, $b) => $a + $b;

   $x = 10;
   $y = 20; 
   echo " x: $x y: $y Addition: " . $add($x, $y);
?>

It will produce the following output −

x: 10 y: 20 Addition: 30

Using the Arrow Function as a Callback Function

We can also use the arrow function as a callback function. Callback functions are used as one of the arguments of another function. The arrow function is executed on the fly and the value of the expression after "=>" becomes the argument of the parent function, which may be either a built-in or a user-defined function.

Example

In this example, we use an arrow function inside usort() function, a built_in function that sorts an array by values using a user-defined comparison function.

<?php
   $arr = [10,3,70,21,54];
   usort ($arr, fn ($x , $y) => $x > $y);

   foreach ($arr as $x){
      echo $x . "\n";
   }
?>

It will produce the following output −

3
10
21
54
70

Accessing Variables from the Parent Scope

Arrow functions can automatically access variables from the parent scope. Unlike the anonymous functions, the "use" keyword is not necessary for it to act as a closure. When a variable used in the expression is defined in the parent scope, it will be implicitly captured by-value.

<?php
   $maxmarks=300;
   $percent=fn ($marks) => $marks*100/$maxmarks;

   $m = 250;
   echo "Marks = $m Percentage = ". $percent($m);
?>

It will produce the following output −

Marks = 250 Percentage = 83.333333333333

Example

Arrow functions capture variables by value automatically, even when nested.

In the following example, an arrow function is defined in the expression part of another arrow function.

<?php
   $z = 1;
   $fn = fn($x) => fn($y) => $x * $y + $z;
   $x = 5;
   $y = 10; 
   echo "x:$x y:$y \n";
   echo "Result of nested arrow functions: " . ($fn($x)($y));
?>

It will produce the following output −

x:5 y:10 
Result of nested arrow functions: 51

Just like anonymous functions, the arrow function syntax allows arbitrary function signatures, including parameter and return types, default values, variadics, as well as by-reference passing and returning.