head

    Returns the head of a list.

function head($items)
{
    return reset($items);
}

head([1, 2, 3]); // 1

tail

    Returns all elements in an array except for the first one.

function tail($items)
{
    return count($items) > 1 ? array_slice($items, 1) : $items;
}

tail([1, 2, 3]); // [2, 3]

countVowels

    Returns number of vowels in provided string.

    Use a regular expression to count the number of vowels (A, E, I, O, U) in a string.

function countVowels($string)
{
    preg_match_all('/[aeiou]/i', $string, $matches);

    return count($matches[0]);
}

countVowels('sampleInput'); // 4

isEven

    Returns true if the given number is even, false otherwise.

function isEven($number)
{
    return ($number % 2) === 0;
}

isEven(4); // true

decapitalize

    Decapitalizes the first letter of a string.

    Decapitalizes the first letter of the string and then adds it with rest of the string. Omit the upperRest parameter to keep the rest of the string intact, or set it to true to convert to uppercase.

function decapitalize($string, $upperRest = false)
{
    return lcfirst($upperRest ? strtoupper($string) : $string);
}

decapitalize('FooBar'); // 'fooBar'

lcm

    Returns the least common multiple of two or more numbers.

function lcm(...$numbers)
{
    $ans = $numbers[0];
    for ($i = 1, $max = count($numbers); $i < $max; $i++) {
        $ans = (($numbers[$i] * $ans) / gcd($numbers[$i], $ans));
    }

    return $ans;
}

lcm(12, 7); // 84
lcm(1, 3, 4, 5); // 60

flatten

    Flattens an array up to the one level depth.

function flatten($items)
{
    $result = [];
    foreach ($items as $item) {
        if (!is_array($item)) {
            $result[] = $item;
        } else {
            $result = array_merge($result, array_values($item));
        }
    }

    return $result;
}

flatten([1, [2], 3, 4]); // [1, 2, 3, 4]

variadicFunction

    A variadic function is a function of indefinite arity, i.e., one which accepts a variable number of arguments. The example shows variadicSum function, that takes an indefinite number of integers and returns their sum.

function variadicSum(...$nums)
{
    $sum = 0;
    foreach($nums as $num) {
        $sum += $num;
    }
    return $sum;
}

variadicSum(1, 2); // 3
variadicSum(1, 2, 3, 4); // 10

compose

    Return a new function that composes multiple functions into a single callable.

function compose(...$functions)
{
    return array_reduce(
        $functions,
        function ($carry, $function) {
            return function ($x) use ($carry, $function) {
                return $function($carry($x));
            };
        },
        function ($x) {
            return $x;
        }
    );
}

$compose = compose(
    // add 2
    function ($x) {
        return $x + 2;
    },
    // multiply 4
    function ($x) {
        return $x * 4;
    }
);
$compose(3); // 20