Examples of function composition using PHP and JavaScript

0.function_composition.php

<?php

function f( $x ) {
  return $x+5;
}

function g( $x ) {
  return $x+1;
}

function fog( $x ) {
  return f( g( $x ) );
}

var_dump( fog(3) ); // 9

1.function_composition.js

function f(x) {
  return x+5;
}

function g(x) {
  return x+1;
}

/**
 * Returns a composite function using two given functions
 *
 * Note that the output of `func2` must be an appropriate input
 * for `func1` whenever the composite function is used
 */
function getComposition(func1, func2) {

  return function(x) {
    
    /**
     * We need an intermediate step since, for some reason, the
     * expression `func1(func2(x))` returns `undefined`
     */
    let placeholder = func2(x);
    return func1( placeholder );
  }
}

let fog = getComposition(f, g);
console.log( fog(3) ); // 9