PHP Exercises: Call a given function only once

Last update on April 19 2025 11:50:09 (UTC/GMT +8 hours)

101. Call a Function Only Once

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:

For more Practice: Solve these Related Problems:

Write a PHP script to wrap a function so that it executes only on the first call and ignores subsequent invocations.
Write a PHP function to cache the result of its initial execution and return NULL for later calls.
Write a PHP script to implement a "call once" mechanism using static variables to track invocation.
Write a PHP script to ensure that a specific function is executed only one time regardless of repeated calls.

PHP Code Editor:

Have another way to solve this solution? Contribute your code (and comments) through Disqus.

Previous:Write a PHP program to curry a function to take arguments in multiple calls.
Next: Write a PHP program to capture a variable number of arguments to a given function.