Composing functions in PHP

compose_functions.php

<?php
 /**
  * Just trying out more functional programming in PHP
  * This gist provides some basic utility functions for:
  *  + Working with functions
  *  + Working with arrays
  */

// This function allows creating a new function from two functions passed into it
function compose(&$f, &$g) {
  // Return the composed function
  return function() use($f,$g) {
    // Get the arguments passed into the new function
    $x = func_get_args();
    // Call the function to be composed with the arguments
    // and pass the result into the first function.
    return $f(call_user_func_array($g, $x));
  };
}

// Convenience wrapper for mapping
function map(&$data, &$f) {
  return array_map($f, $data);
}

// Convenience wrapper for filtering arrays
function filter(&$data, &$f) {
  return array_filter($data, $f);
}

// Convenience wrapper for reducing arrays
function fold(&$data, &$f) {
  return array_reduce($data, $f);
}

// f(x) = (1/2)*x
$f = function($x) {
  return (0.5*$x);
};

// g(x) = x^2
$g = function($x) {
  return ($x*$x);
};

// h = f . g
$h = compose($f, $g);

// Get a range of values
$X = range(0, 10);
// Print the application of $h of $X
print_r(map($X, $h));