functional.php

<?php

function take($num, $iterable) {
    $i = 0;
    foreach($iterable as $key => $val) {
        if($i >= $num) {
            break;
        } else{
            $i++;
            yield $key => $val;
        }
    }
}

function map(callable $function, $iterable) {
    foreach($iterable as $key => $val) {
        yield $key => $function($val); 
    }
}

function filter(callable $predicate, $iterable) {
    foreach($iterable as $key => $val) {
        if($predicate($val)) {
            yield $key => $val;
        }
    }
}

$iter = take(6, fib_gen());
foreach($iter as $v) {
    var_dump($v);   
}
/**
 * int(0)
 * int(1)
 * int(1)
 * int(2)
 * int(3)
 * int(5)
 */
 
$double = function($i) {
    return $i * 2;
};
$isOdd = function($n) {
    return $n % 2 !== 0;
};

$iter = map($double, filter($isOdd, take(10, fib_gen())));
foreach($iter as $v) {
    var_dump($v);
}
/**
 * int(2)
 * int(2)
 * int(6)
 * int(10)
 * int(26)
 * int(42)
 */