PHP Exercises: Call a given function only once

PHP: Exercise-101 with Solution

Write a PHP program to call a given function only once.

Sample Solution:

PHP Code:

<?php
// Licence: https://bit.ly/2CFA5XY

// Function definition for 'once' that takes a function as a parameter
function once($function)
{
    // Return a closure that encapsulates the original function
    return function (...$args) use ($function) {
        // Static variable to track whether the function has been called
        static $called = false;

        // If the function has been called before, return early without executing the original function
        if ($called) {
            return;
        }

        // Mark the function as called and execute the original function with the provided arguments
        $called = true;
        return $function(...$args);
    };
}

// Anonymous function definition for 'add' that takes two parameters and returns their sum
$add = function ($a, $b) {
    return $a + $b;
};

// Create a 'once' version of the 'add' function
$once = once($add);

// Call the 'once' version of 'add' with arguments and display the result using 'var_dump'
var_dump($once(10, 5));  // Output: int(15)

// Attempt to call the 'once' version of 'add' again with different arguments
// The second call should return early without executing the original function
var_dump($once(20, 10)); // Output: NULL
  
?>

Explanation:

• Function Definition:
• The once function is defined to accept another function as its parameter.
• Closure Creation:
• Inside once, a closure is returned that captures the original function using the use keyword.
• Static Variable:
• A static variable $called is initialized to false to track whether the original function has been executed.
• First Call Check:
• The closure checks if $called is true. If it is, the function returns early without executing the original function.
• Marking the Function as Called:
• If the function has not been called before, $called is set to true, and the original function is executed with the provided arguments.
• Anonymous Function for Addition:
• An anonymous function $add is defined to take two parameters and return their sum.
• Once-ified Function:
• The once function is called with the $add function, creating a version of $add that can only be executed once.
• First Function Call:
• The once version of the add function is called with arguments 10 and 5. The result (15) is displayed using var_dump.
• Second Function Call:
• The once version of the add function is called again with different arguments (20 and 10).
• This call does not execute the original function, as it has already been called once, resulting in NULL being displayed.

Output:

int(15)
NULL

Flowchart:

PHP Code Editor:

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