Why does the function signature differ between array_map and array_filter/array_reduce?

Question

array_map asks for the $array input as the last parameter(s). array_filter and array_reduce take $array input as the first parameter. As a contrasting example, when you call map, filter or reduce on an array in JavaScript, the callback function signatures look like

(current, index, array) => {…}

Array.prototype.reduce takes the carryover value as the first parameter, but it is still impossible to mix up the parameters' order in the JavaScript methods.

I know that PHP isn't functionally-oriented, but I'm wondering what the design decisions were leading to the signatures for array_map etc.

Does array_map take an array as the last parameter simply because you can feed as many arrays as you want (variadic)? Does the ability to feed an arbitrary number of arrays through the callback function of array_map outweigh having more uniform function signatures?

EDIT/ Comment:

from Wikipedia, this puts in perspective just how long PHP has been evolving:

'Early PHP was not intended to be a new programming language, and grew organically, with Lerdorf noting in retrospect: "I don’t know how to stop it, there was never any intent to write a programming language […] I have absolutely no idea how to write a programming language, I just kept adding the next logical step on the way." A development team began to form and, after months of work and beta testing, officially released PHP/FI 2 in November 1997. The fact that PHP was not originally designed but instead was developed organically has led to inconsistent naming of functions and inconsistent ordering of their parameters. In some cases, the function names were chosen to match the lower-level libraries which PHP was "wrapping", while in some very early versions of PHP the length of the function names was used internally as a hash function, so names were chosen to improve the distribution of hash values.'

Just as a quick note, array_map, array_reduce and array_filter were added into PHP in 4.0.6 (released in 2001). It looks like Array.prototype.reduce was introduced in ECMAScript 5.1, which was published ten years later (in 2011). — Chris Forrence
– Chris Forrence, Commented Jun 2, 2016 at 22:21
That's a thing almost every developer brags at first, when confronted with PHP. It's inconsitency in naming and argument-order of most (at least older) utility-functions. It's a matter of getting used to it and learning the function-names and their arguments, since you can't change it anymore, or you'll break half the internet (at least). On the other hand I think PHP has one of the best Documentations (to work with). — Thomas
– Thomas, Commented Jun 2, 2016 at 22:50

Chris Forrence · Accepted Answer · 2016-06-02 23:02:01Z

Does array_map take an array as the last parameter simply because you can feed as many arrays as you want (variadic)?

Yes; only the last parameter in a method signature can be variadic.

Does the ability to feed an arbitrary number of arrays through the callback function of array_map outweigh having more uniform function signatures?

Essentially, this is asking if array_map should accept multiple arrays. There are good use cases for allowing for calling array_map with multiple arrays; below is an example from the PHP array_map guide that I've slightly modified:

<?php
function showLanguages($n, $s, $g)
{
    return("The number $n is called $s in Spanish and $g in German.");
}

$a = array(1, 2, 3, 4, 5);
$b = array("uno", "dos", "tres", "cuatro", "cinco");
$c = array("eins", "zwei", "drei", "vier", "funf");

$d = array_map("showLanguages", $a, $b, $c);
print_r($d);

What if array_map were changed to have a multidimensional array as the first argument? While it would technically make the function signatures more uniform, it would actually add some additional problems.

The underlying code would not only have to validate that the first argument is an array, it would have to validate that it's an array of arrays. That would make it essentially be a different kind of parameter than just an array, so it would still be a different method signature than the other array_* methods.

If you really wanted to have the callback as the last argument, you could use a modified function that removes the last element, move it to the first argument, then call array_map on that. You would lose all type-hinting, though, so it'd be harder to use.

/*
 * Accepts a variable number of arrays as the first N arguments, takes a callback as the last argument
 */
function map()
{
    $args = func_get_args();

    $callback = array_pop($args);

    array_unshift($args, $callback);

    return call_user_func_array("array_map", $args);
}

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