fib_gen.php

<?php

function fib_gen() {
    list($a, $b) = [0, 1];
    while(true) {
        yield $a;
        list($a, $b) = [$b, $a+$b];
    }
}

$fib = fib_gen();
var_dump($fib->current()); # int(0)
$fib->next();
var_dump($fib->current()); # int(1)
$fib->next();
var_dump($fib->current()); # int(1)
$fib->next();
var_dump($fib->current()); # int(2)
$fib->next();
var_dump($fib->current()); # int(3)
$fib->next();
var_dump($fib->current()); # int(5)